Also experiencing this issue on both PS4 and PS5 Versions. Both accounts have PSN and Microsoft accounts properly linked. To verify the issue one of the linked Microsoft accounts used a different device and was able login and join the realm as a 3rd. So we had “User1”, “User1(1), and “User2” (User2 being the different device login). It should just be “User1” & “User2; User2 should get rejected when attempting to login from the other device. It appears that split screen user are not using their Minecraft accounts and are treated as a guest entity.
Canceled our realm membership until this gets fixed. We had to swap to a locally hosted world. Hope this gets fixed in the future.