I wish I could help you but it seems like you did everything right.
Just to make sure:
- You created a map and did whatever you did
- You pressed "Escape"/"Start" and selected "Save Mission"
- You went back to the mission select and there is no new level named however you named it right next to the "Create New Level" in "The Mole's Workshop"

Have you tried to verify the game files?
And you could check if there is a .lvl .png and .xml file named like your mission in Steam/userdata/"Your ID"/113020/remote/world.

That's everything I can think of regarding your problem but it seems like you're not the only one who can't save maps so I hope that one of the devs can help you better.

Thanks for the reply!

I did it just like you stated:
-created map;
-escape > "Save Mission";
-went back to mission select and found no new level next to "the mole's workshop";

However, I did find the files on the "Steam/userdata/"Your ID"/113020/remote/world" as you suggested! But they won't do me much good since I have no way of opening them with the editor.

That's true, they won't do you much good like this but atleast they exist, so it looks like your map was actually saved and it's only a problem with it showing up.
Sadly I have no idea at all on how to fix this :(