After update: Plex only works as admin on Synology despite having permissions

Do you mean this?

The only thing I maintain is the Plex share itself. It is for exclusive use with PMS data. Media doesn’t go in there.

What do you mean by “Only works as ‘admin’” ? If you had to put it back as it was because you didn’t apply the share permission changes then I suggest you revert to non-privileged and allow the DSM mechanisms to work as intended.