Also had the same problem today, specifically with Halsin when moving through the Rosymorn Monastery area. Unsure if it first happened after a combat, splitting my party in the same zone (jumping to different areas of the monastery), or from using group stealth (was doing all before I noticed) but anything I tried meant Halsin wouldnt follow the group after the bug began. If Halsin was leading, all other companions would follow, but he would never follow others.
As above, I tried:
Unchaining and rechaining
Toggling Group Mode On/Off
Short/Long Rest
Fast travelling locally and to another region
Removing Halsin for another companion in camp, then reinviting Halsin after a rest
Triggering dialogue
I then went to camp and dropped him for Astarion who followed fine, then swapped back and Halsin, who still refused to move.
In the end I just reloaded a save from a couple of hours before to remove the bug.
A Google search led me to an old (locked) thread on these forums from Early Access, which has some more details:
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=818924
