Same. Workshop shows they are subscribed, but they don't show in game. and I can't add any, they likewise don't show in game.

why is this happening? I was playing fine yesterday, now I can't use any mod

Same here. But 'unsubscribe' and 'subscribe' again works till a certain level. Still some mods gave problems (I have to try to un- and subscribe them again).

I've been having the same problem, which I've been solving by unsubscribing, resubscribing, and migrating the affected files to my local folder then unsubscribing them from the Workshop.

I don't know if this is a Steam issue or a CO issue, but in any event, it's extremely annoying.

It happens almost every week. I start the game from Skyve, also use Loading Screen Mod to track down any issues. Skyve shows exact number of mods and objects downloaded to hard drive and it is obvious that Steam background process can randomly delete folders from hard drive. It is super annoying becasue it takes a lot of time to start the game, check which mods got deleted, close the game and resubscribe them, start the game again and load save, then check LSM report for missing assets. If you don't save your progress in new file every time you risk data loss because game doesn't store information of missing buildings when asset's local folder is deleted. This is based on my personal experience. Literally anything can be removed from hard drive: a network, decal, building or vehicle, even mod, despite being subscribed all the time. For example, I'm subscribed to 72 mods now but Skyve reduced this number to 69. Additionally, a few assets disappeared from workshop local folder. This is why when I loaded my save today I got simulation error and couldn't load save. Ideally, Steam might allow users to play offline with files sync turned off.