Hello @Gladox114, this issue is already being tracked at https://github.com/ValveSoftware/steam-for-linux/issues/7080 / #428 / #231. Closing in favor of the older issue reports.
EDIT: It already got mentioned https://github.com/ValveSoftware/Proton/issues/428#issuecomment-610331246
Issue https://github.com/ValveSoftware/steam-for-linux/issues/7080 and mine are very similiar or even the same.
Wouldn't it be easy to fix this problem by just merging the demo version prefix and the full game prefix. Or merging the steamuser folder in both prefixes because in windows they would stay in the same place meaning that it shouldn't cause problems.
While https://github.com/ValveSoftware/Proton/issues/428 / https://github.com/ValveSoftware/Proton/issues/231 are maybe not as that easy fixable like my situation and his ValveSoftware/steam-for-linux#7080
proton experimentalx1 2020-12
Bug Report
System Information
I confirm:
Symptoms
Characters made in Demo version of CODE VEIN are getting lost during full buy and install of the game
Reproduction
Start new game and create Appearance (Character) in the Demo version.
Buy and install the full game.
CODE VEIN demo isn't in your library anymore (Normal)[Steam probably wipes the prefix where the saved data is located, this is bad]Start new game and check if
Load Character's Appearance from Demois available.