Similar problem here. While Windows-hosted Plex Media Server photo libraries appear fine, Linux-hosted photo libraries are completely missing, despite working ok in web app and the old Plex mobile app.
I’m running a Plex server on Ubuntu 22.04/Raspberry Pi. I’ve tried both snap and apt repos, to no avail.
Try checking your local LAN for adware blockers like PiHole and whitelisting the used hosts.
When I disabled DNS blocking or whitelisted the sites (being blocked) for the Plex Photo App, it started to show my libraries for me.
Still the app in total is a piece of crap so far. A major downgrade from the functionality inside the main Plex app.
No landscape view of images/videos.
No seeking inside played videos.
No details shown on the folders or images. (like path on the server etc.)
No option to reload/rescan media libraries.
Everything that worked perfectly on the main (old) app.
So the person conciously deciding to destroy the main Plex App (taking functionality out) and replacing it with this unworthy piece of software should really rethink his job position.
After all the well functioning Android App, at least for me, was a major reason to buy a license for the Plex Server.