Now that patch 6.1.5 is released, I tested this by traveling to the Alikr Desert and joining a public dolmen farming group.
I was immediately booted with error 318, exactly as before this patch.
Hopefully the "fix" released today helps with the occasional failures joining groups of size 4 (!) and guaranteed failure joining groups of size 12. I don't have much hope for Cyrodiil if a dolmen farming group is too big for ESO Stadia.
I started playing ESO on my laptop+external monitor thanks to Stadia and am awfully hooked, but it looks like for these activities, I'll have to dig out my potato PC with a large hard drive.