I can think of a few ways how this could happen:

• the same folders exist in "Documents/Paradox Interactive/Europa Universalis IV". Their contents will override all mods
  
• there are other mods active which override the files of your mod
  
• there are other mods active which break the files of your mod
  
• your Documents folder gets synced with a cloud service (e.g. Microsoft OneDrive) and the files of your mod are in a not-synced or offline state which prevents eu4 from accessing them(but your file manager can still access the files).
  
• like above, but it is some kind of disk-space-saving application

Originally posted by grotaclas:

I can think of a few ways how this could happen:

• the same folders exist in "Documents/Paradox Interactive/Europa Universalis IV". Their contents will override all mods
  
• there are other mods active which override the files of your mod
  
• there are other mods active which break the files of your mod
  
• your Documents folder gets synced with a cloud service (e.g. Microsoft OneDrive) and the files of your mod are in a not-synced or offline state which prevents eu4 from accessing them(but your file manager can still access the files).
  
• like above, but it is some kind of disk-space-saving application

I haven't found a fix but I've ruled out the first three. If it's the fourth it's likely an issue with connecting to the steam cloud, I don't think it's the fifth since I've uninstalled all disk saving applications but just to be sure I'll do a fresh windows reinstall as a last resort.

The steam cloud is not responsible, because it does nothing with your Documents folder. It syncs save games in cloud sync folders within the steam folder, but it does not touch the eu4 folder in Documents. Steam could have an impact on steam mods, but it won't touch your local mods. Do you have Microsoft OneDrive? It is active per default on windows

But maybe I misunderstood your problem. Is any part of the mod still active in the game? Or does it not work at all? If it does not work at all, there is another explanation: You have another .mod file which has the same name= line. Eu4 can't handle multiple mods with the same name and at most one of them will work.

Originally posted by grotaclas:

The steam cloud is not responsible, because it does nothing with your Documents folder. It syncs save games in cloud sync folders within the steam folder, but it does not touch the eu4 folder in Documents. Steam could have an impact on steam mods, but it won't touch your local mods. Do you have Microsoft OneDrive? It is active per default on windows

But maybe I misunderstood your problem. Is any part of the mod still active in the game? Or does it not work at all? If it does not work at all, there is another explanation: You have another .mod file which has the same name= line. Eu4 can't handle multiple mods with the same name and at most one of them will work.

I've had OneDrive disabled and I don't use other cloud services.

The mod isn't active ingame, even when the mod is enabled in the launcher.
Changing details of the descriptor like the name= or version number is no longer updated in the launcher and yet the launcher does not give any kind of indicator that the file cannot be found.

I've been sure no mods in the documents folder share the same name= line, I keep all of the test mods with the same name= in a separate folder outside of the EU4 and steam directories.

Originally posted by Pluri:

.
The mod isn't active ingame, even when the mod is enabled in the launcher.
Changing details of the descriptor like the name= or version number is no longer updated in the launcher and yet the launcher does not give any kind of indicator that the file cannot be found.

Where do you make the changes? The descriptor.mod file in the folder of your mod should be the primary file. The launcher should use that file to automatically update the .mod file in "Documents//Paradox Interactive/Europa Universalis IV/mod". You could try the button to reload all mods in the all installed mods list of the launcher to make it check again. Or maybe something prevents the launcher from properly reading and updating the files. But this should give some kind of error message.
What is the full path to your Documents folder? Does it maybe contain any non-latin characters like ã, ä, å, æ, ç, ñ ?

Originally posted by grotaclas:

Originally posted by Pluri:

.
The mod isn't active ingame, even when the mod is enabled in the launcher.
Changing details of the descriptor like the name= or version number is no longer updated in the launcher and yet the launcher does not give any kind of indicator that the file cannot be found.

Where do you make the changes? The descriptor.mod file in the folder of your mod should be the primary file. The launcher should use that file to automatically update the .mod file in "Documents//Paradox Interactive/Europa Universalis IV/mod". You could try the button to reload all mods in the all installed mods list of the launcher to make it check again. Or maybe something prevents the launcher from properly reading and updating the files. But this should give some kind of error message.
What is the full path to your Documents folder? Does it maybe contain any non-latin characters like ã, ä, å, æ, ç, ñ ?

I make the edit to the descriptor file within Europa Universalis IV/mod/mod_name but the descriptor within Europa Universalis IV/mod does not update.

Manually changing the descriptors to match each other causes the mod name within the launcher to update but does not fix the issue causing the mod to not activate ingame.

I tried reloading all the mods and clearing out the cache just in case, sadly it doesn't seem to have fixed the issue.

No error messages in both tests, launcher still says the mod is detected and enabled.

The full path is:
PC_Name > Documents > Paradox Interactive > Europa Universalis > mod > mod_name
No non-latin characters.

Originally posted by Pluri:

The full path is:
PC_Name > Documents > Paradox Interactive > Europa Universalis > mod > mod_name
No non-latin characters.

That's what's showing up in the windows explorer, but that's not the actual path. A path on a local drive starts with a drive letter followed by :\\

You could check the path= line in the "Europa Universalis IV/mod/mod_name.mod" file to see where the launcher expects the mod. You could also try to change that path to "mod/mod_name", so that it is relative and not absolute. This should avoid issues with wrong characters in the full path. But the launcher will likely change it back. Also make sure that the descriptor.mod file in the folder of your mod does not have a path= or an archive= line, because they could mess things up

Originally posted by grotaclas:

Originally posted by Pluri:

The full path is:
PC_Name > Documents > Paradox Interactive > Europa Universalis > mod > mod_name
No non-latin characters.

That's what's showing up in the windows explorer, but that's not the actual path. A path on a local drive starts with a drive letter followed by :\\

C:\Users\User_Name\Documents\Paradox Interactive\Europa Universalis IV\mod\mod_name

is your username really User_Name? Or does it maybe contain any special characters?

Originally posted by grotaclas:

is your username really User_Name? Or does it maybe contain any special characters?

No, it's just a stand-in for the username. I found a fix though!
I checked the path= line like you suggested and it was missing for some reason, so I added it back in.
I'm not sure why it was missing, it was definitely there when I created the mod.
I would assume I just made edits and forgot.. I wouldn't really have reason to edit that file though since it's the automatically updated descriptor and yet it seems to have happened several times to different local mods.
Thanks for the help.