If I make a parent constraint in FileA, and then I reference FileA into another scene, sometimes that parent constraints work and sometimes they don't. It seems random.
I can have the actual FileA open and confirm that the parent constraint does work as expected. And in the other file where FileA is being referenced, everything appears identical, yet the constraint doesn't work.
This has to be a bug? The workaround is simply that I setup constraints in the animation scene rather than the rig file which gets referenced into animation files. It's not a big deal, but I just want to double check I'm not missing something obvious. It would be nice to setup constraints once and not think about it again.