Your system information

• Steam client version (build number or date): 1535062307
• Distribution (e.g. Ubuntu): Ubuntu 18.04.1
• Opted into Steam client beta?: [Yes/No] Yes
• Have you checked for system updates?: [Yes/No] Yes

Please describe your issue in as much detail as possible:

I loaded my old save, when i played in Grim Dawn with Steam Play feature and after exit all of my achievements was reset.

Steps for reproducing this issue:

1. Start Grim Dawn with Proton 3.7
2. Exit game
3. All your achievements will be reset

I'm not sure is this Steam Play(Proton) bug or Steam for Linux so take a look:
You can see that my playtime is enough to gain at least one achievement.
My Steam page
Video with bug reproduced