I’ve been seeing this bug off and on with my workbench thralls and follower thralls.
Many times, but not enough to be a bother. I just remove and place the thrall again and it’s solved. It’s kind of freaky, though! 
+1 
Yes, it even happens with the slaves that are following you.
I usually just say they’re slacking off at work and I shout at them… then I pick them up and place them back. Back to work!