After beating the Guardian in a GR, and after speaking to Urshi, but BEFORE speaking to Orek to close the GR, I can’t change followers.
I can change their gear, sell items, fully access the stash, change my gear, and dismiss my follower. But I can’t switch to a new follower, nor hire a new follower after dismissing my current follower.
Everything works as expected after I speak with Orek.