I managed to downgrade to 3.5.0.beta1 and refreshed all my browser caches etc. But I am still facing this problem.
It’s extremely weird, I know I was on a recent version, less than 2 weeks old. It worked there, I set up 2 channels with auto-adding people, when I said “yes” to add people from an existing group that all went through perfectly.
Now, I tried again and it doesn’t work properly, except for 1 channel. Just to illustrate:
All of these should have 6 or more members. When there’s 1 member, it’s just me. The “Diversity equity and inclusion team” there is the only one that consistently adds all the members of the group.
The “Packages team” on my live site, worked beautifully when I created it, all 10 people were added. It worked on the dev site as well before. Now when I remove it and add it again on the dev site, it consistently only gets 3 members added.
I have compared the groups and all the group settings are exactly the same. I removed and added people in groups, that didn’t make a difference either.
Finally, when I add a user to the group that had never been part of that group before, it seems to add it to the chat channel. But when I remove and add an existing member, the same doesn’t happen.
I even deleted the chat channel, removed people from the group, added the chat channel again and added the old users back to the group. That didn’t work. Adding a random other user in.. DOES work.
Maybe something is completely messed up in my instance.. I don’t have a chance to test it in a fresh instance.
I am happy to give some SSH access and admin access to the dev site, it’s a dev site so I’m fine with messing up anything and everything

Edit: upgraded now to the very latest commit from a few hours ago. I tried to add a brand new group (with the same people in it) to an existing private category and see if that would help, but same result, nobody gets added to the chat channel.