Issues joining remoteplay with friend

[Babayaga]

Bought Steamdeck for friend so they can play via remote play together for couch coop games since they live far away. Works, very poorly. I've played loads of 'remote play together games' with other people on PC's without any issues other than some network latency which is normal. My son also has an older first gen Steamdeck, which always seems to accept invites to play something together without any problem within the same network/home. The Steamdeck having an issue is brand new, with a brand new account. This user is currently in my home for the holidays and we're having the same issues locally as they were having remotely.
A purchase was made to get past the $5 minimum we found that would set the account to limited / restricted, this changed nothing.
Also tried to just 'watch' from their steamdeck, it will sometimes work, but usually just stays at "loading..." screen.
They also have a dock, being docked or undocked makes no difference.

Whenever the steamdeck user gets an invite to play together, it joins the game, audio can be heard, but cannot open the game or switch to wherever it's playing from on the deck.

What does work:
Starting game from Steamdeck, invite remote friend, (not a steamdeck), and things usually work normally. Unfortunately the user on the steamdeck doesn't own much of anything.
I tried matching all settings from my son's older steamdeck (which WORKS!) and there is no change.
Switching to desktop mode on steamdeck, resending invite from PC user, accept invite. This will usually work, but it's a very clunky work-around.

I just want to be able to invite the person on their steamdeck to play a game, they accept, and we play. This doesn't work.

Any suggestions that aren't one of the general solutions posted on every Internet search result? (I've tried em all).

EDIT: About 20 different games that are definitely remote play together capable have been tried.

[renickdp]

same issue, cant play with 2 steamdecks together happened after last update