Steam Creates and Forces the Use of "Phantom" Users When Launching Games Played by Two Other Steam User Accounts

[ThisNekoGuy]

Your system information

• Steam client version (build number or date): 1747701111
• Distribution: Gentoo Linux (LLVM)
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes
• Steam Logs: steam-logs.tar.gz
• GPU: AMD Radeon RX 7800 XT

Please describe your issue in as much detail as possible:

When sharing my Linux account (not my Steam account) between two people on the same machine, and playing the same game per separate Steam accounts (we're both also Steam Family members), trying to play the game a 3rd time effectively locks us both out of our own account for that game specifically.

For example:
Me -> Them -> Them
- or -
Me -> Them -> Me

Triggers a phantom "third" user to be created by Steam that's then directed to the affected game to be used.

What's really bad about this problem is, not only does it lock us out of our own save data for the launched game, it also causes paid DLCs not to validate because of these false "third" users (I've had this happen with two games).

I originally tried reporting this here, with more details, but I realized where I mentioned it was technically a different issue and that this problem deserved its own.

To quote what I said over there:

I just found both of our save data in the compat folder (game is Romancing Saga 2, in this case), and it is actually sorted by ID but a new folder with the ID 00000000000000000 appeared with init save data from the game, as if the game was launched for the first time

Deleting or removing proton prefixes affected by this doesn't seem to fix it either, at least not for SoulCalibur VI

For Romancing Saga 2: Revenge of the Seven, it gets represented as repeating 0s but, for SoulCalibur VI (a game with DLCs), it got represented as: GENTOO-F730A1F64CCE37DE1857C28CE31FEE71 for some reason.

I did manage to fix it for Romancing Saga 2: Revenge of the Seven once by removing both the bogus SteamID folder and the legit "User 2" folder, but the problem returned after I launched the game twice as myself:

Delete UserID save-data folders -> Launch game as myself (works again) -> Close the game -> Launch the game again (Problem came back)

Also, to be clear, once it came back again in that case I couldn't get it to stop again, so I really don't understand the method to this madness.

Steps for reproducing this issue:

1. Login to User 1, and play a game with Proton (for this example, Romancing Saga 2 or SoulCalibur VI) - Use Proton 10
2. Create save data if there isn't any already, then exit
3. Log out of User 1
4. Login to User 2
5. Launch the same game (with Proton 10) and repeat step 2, if applicable
6. Either log out and launch the game again as User 1, or do so again as User 2 - it doesn't matter

[kisak-valve]

Hello @ThisNekoGuy, we're tracking issues with games run with Proton in the Proton issue tracker, so I've transferred this issue report.