I have never seen this happen before. I don’t know for sure but it really feels like this happened because of the name. I would avoid using periods “.” in album names. Possibly that is what caused the conflict. Did you log into Lightroom on the web at lightroom.adobe.com. What do you see there?
Thanks Theresa, I decided to go down the route that I feared I may have to initially, so I deleted the album, then deleted the photos themselves from "All Photos", recreated the album and added the photos back in, and everything appears to be syncing correctly now.
Photos are now showing in the album across all devices.
Side note, the album is called "sorasu.jp 2" like it was previously and everything is syncing correctly so doubt that it was a naming error?
I'm really not sure what the root cause of this actually was.