I completed some campaigns in 2020/2021 and have the Steam achievement for that, but somehow in game I lost the progress/medals after reinstalling the game at some point.
Is there a way to get the campaign progress back?
Do you have the Steam Cloud enabled in the properties tab of your game? Maybe that’s where your issue stems from, if your user profile got deleted locally and your progression wasn’t tracked via the Steam cloud.
On the bright side of things: you’ve got the perfect excuse to play again all the campaigns! 
I have it enabled. 20 chars
It might be possible that your Steam Cloud was full and those saves from campaigns were stored locally, are you using another pc?
I’m pretty sure my steam cloud was and is full from autosave on disconnect.
edit: cloud capacity is at 99.90 percent like forever…