Many years of teaching archery and working on bows of all kinds gives me a working knowledge of common problems and mistakes. Granted sight unseen makes it a little more difficult, but his description of the problem was fairly detailed and his POI shift direction is one I have seen several times. Almost every one of those times was from a problem with the rest. Him letting us know it is a WB pretty much clinched it for me. And there are many bow shops out there that will just give a bow a cursory glance and see that everything is tight and not be able to figure out the problem.

For instance, the old TM hunter style rests had a common problem that would create a similar shift because of spring tension wear. And it would sometimes suddenly pop up verses a gradual thing. Sometimes the adjustment screw for the prong height would SEEM tight enough but firing would cause a slight shift in some models.

With the stupid design of the WB, you have wear in the bristles. ESPECIALLY from most carbon arrows in higher speed bows. I'm betting he has enough wear in the bristles that it raised his POI. It probably went unnoticed by the guys in the shop.

Granted this is just an opinion but it's an informed opinion stated from many moons of experience with a common problem. It's extremely rare that you get a POI shift over night from an unconscious form change that stays consistent even after you shift your sights to the POI change. Muscle memory just doesn't act like that. And he sounds like he has been shooting long enough to gain a pretty fair amount of muscle memory.