Hi Admin. Very unusual, imho. You could try putting a dead straight edge against the head tube, at the front,between the bearing cups, to check that the head tube has not warped or bent. Sight for daylight between the straightedge and the tube. But best you take your frame or complete bike to a framebuilder for further checking. e.g. the ends must be cut dead parallel to each other; the headtube should be checked that it was reamed at both ends, again, square on and dead parallel. The steerer tube of the front fork may need to be checked in case it too could be bent or warped. The new headset may have been overtightened on installation,
this is the most likely cause. Also, the mechanic could be installing the new cups using a length of threaded rod and a set of old bearings to pull the new cups into place. This is a sure way to literally give the new cups the pits. The tool for quality results should have bits of machined metal, formers, which will fit the cups exactly and pull them down without any distortion whatsoever. But definitely have that headtube checked for proper and true reaming. Sounds like you need better service, if this problem is so soon repeated. Best.