well i have similar issue, on two macs now that i've completed the upgrade to sonoma 14.5 (yesterday for my m2 pro mac mini)
So if you have a finder folder with photo's and you set them to rotate randomly say every minute or 5 minutes, or so on, it always takes like 50 seconds or so after the next minute or next 5 minute on the clock has gone, for it to fade to the new photo, while in ventura it was spot on with the system time snap on each 1 or 5 minute or 15 minute or whatever interval
I would suggest filing a Feedback with Apple about it, and include the sysdiagnose files and stuff. If you could reference FB13282770, that would also be super awesome. FWIW, not on ANY version of Sonoma has this worked for me-
Set Desktop Wallpaper to cycle thru a folder in the Photos app, whether it's set to Random or not. It only shows the Sonoma default Wallpaper.
I AM, though, able to set a Finder Folder to cycle thru for Desktop Wallpaper, both Randomly and not. Although I do see this weird issue where the Wallpaper will very quickly switch to another Wallpaper, but then fade into yet ANOTHER Wallpaper. Happens each & every time.
I'll also note that at one point about month+ ago or so, I had a completely unrelated issue and had to have my M1 MBP sent in for repairs, so it had to be wiped. Ended up setting up again by Migrating from a Time Machine backup, and the issues continue. I have this same issue with my 2018 Intel Mac mini, so I'm not sure what's going on. Doesn't matter if I choose a folder in the Photos app with 10 photos or 10,000+, it's the same result.