Cannot change preferred copy of Steam Family game once all DLCs are common

[Jonathing]

Your system information

• Steam client version (build number or date):
• Distribution (e.g. Ubuntu): Nobara Linux 40 (KDE Plasma)
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes
• Steam Logs: steam-logs.tar.gz
• GPU: AMD Radeon 7700S (Framework Laptop 16 dGPU)

Please describe your issue in as much detail as possible:

Steam Family is great, but there's an issue with the "change preferred copy" component. Here is my precise issue: I recently purchased all the remaining DLC for Darkest Dungeon, a game my family member owns. However before that, I was using his copy of the game since he already owns all DLC. Now that we have common DLC, there is no need to change the preferred copy. However, Steam still has the perferred lender of the game set to my family member, and I have no way to change it.

• I poked around in Steam's console and found no commands for Steam Family (reasonably so, as it involves sharing literal game copies)
• We can no longer play the game at the same time as I have no way of changing the preferred lender.
• Steam is very clearly capable of doing this, as when I change the preferred copy of Civilization VI (another game we have in common but with differing DLCs), that goes through. This can be seen in the librarysharing_log.txt log.

This is something I have gone to Steam Support for, but they turned me away when I told them that I was running the Steam Client on Nobara linux, which is not Ubuntu (not officially supported). Coming here is my last resort.

Steps for reproducing this issue:

1. Join a Steam Family and make sure at least two people have the same game but different DLCs.
2. Buy the remaining DLCs into the account that does not have common DLCs.
3. You are no longer able to change the preferred copy back to your own.