I have been like this during the last 6 months waiting for the team to solve this issue. Is anyone at any point experiencing the same? Or do we know if plex team is trying to solve this issue?
I have been doing a bit of research and it seems like the setting of the user are saved no where, which means that everytime a user logs into a different computer, he needs to create new config, also if user logs out in PMP and logs so that another user can log in and then relogs in again, the settings will not be neither saves. Do not really know why this is not solved and all user config is synced between devices and stored into the account data as password and other settings of the account.
You have to change this for each client you use and after the change, make sure you hit the SAVE button at the bottom of the quality setting. I switched to the other home user (wife) and made a change there as well then went back to mine and it kept.