I have also started noticing this. What i found is that main account 1 and spare acount 1 need to be in the same lobby then launch a session via uefn. When main account 1 gets the loading screen that says connecting then insta press join on the spare acount 1 it goes in most of the time but sometimes can just stay on connecting screen and never joins. If main account 1 is in the lobby and it says finding game or whatever it is now then if spare account 1 presses join it just keeps saying finding game/server found even though it was found. This is a very weird bug for sure.
If main account 1 is in a game already then spare account 1 cannot jip it just stays on connecting screen forever so to get this working you need to do restarts and mess about for sure