I can't believe I hadn't registered here earlier. Wanted to be sure to share my experience with this issue since patch 8 update on Stadia.
Shadowheart is currently being salty to the rest of us and stopped following. Issue for me started after using a combination of group sneak, separating members, having some in sneak, others not....then changing my mind and toggle regroup and toggle group sneak on/off so everyone gets on same mode. That's when the matrix glitch happened. I noticed it slightly when group sneak wasn't working for everyone, even tho full party were linked together. I ended up clicking on shadowheart and maybe a few others to toggle sneak manually. Shortly afterwards, my companions had a brief moment of lagging behind MC moreso than before. Then noticed shadowheart was not following I and being impacted by the same issue others have shared.
1. Shes shows as connected and linked with whole party, but doesn't move.
2. Setting her as leader or selecting her as current character has the other 3 members follow her. Setting any of the other 3 as selected character she will not follow the rest of group.
Nothing in-game will resolve the issue or fix it. If you continue to play it may introduce more bugs and issues. Like the character acting weird and glitchy at camp. I saw posts where it could start impacting more characters....like covid! I haven't tried loading previous save file yet, which I think may be the current workaround for now.
Instead...Im playing a little bit more of my current playthrough. ....Not recommended.
IE. - I swapped out shadowheart for Wyll. The issue wasnt apparent. Thought I was in the clear. Nope....now Wylle has an imp familiar that died but still shows pic frame next to him. So I camped and was still showing. Of course I tried casting it again and now have 2 imp pic frames. 1 dead, 1 alive. I can still double click on the dead ones frame and it takes me to it's buriel site in a different area.
Last thing I'm doing with this current playthrough is continuing the story using only 3 total party members. Only for a lil bit tho.