So I grind and I grind get to the last couple single player campaign missions and my save file is... Gone. Just gone. Re-launched the game 5 times. Gone.

Anybody out there know how to fix this?

Have you looked if they are still in the Save-folder?

Did you maybe accidentally switch from the Enhanced Version to the Classic Version or vice-versa?

If all else fails, Warcraft 2 provides cheat codes with which you can jump to a specific scenario.

Another possible (in theory) problem is if you install war2 into Program Files directory instead of the default path.
Then saves can be moved into virtual store. so need to find them.

I have identical problem with many old games like Diablo (original) and solution to this is:
a) run game as admin
b) give game proper authorization to write in the game folder

usually a) is simplies if you don't know how to do b)

I think that gog version should be without this bug, but who knows.

I recommend never installing games in program files due to this reason. Games that store savefiles in the game folder won't work correctly. You should see them back if you give admin rights to the launcher/game exe, or just move the game elsewhere.