Unusual wear on left front tire.

We have 2017 Prius 3 touring. 135k miles. The car is wearing washboard on the inside of the left front tire. We have the car maintained at the dealer. all regular checks, rotations and alignments are accomplished at required intervals. When the tires are rotated the new left front will do the same thing after each rotation. We have had the Dealer look into this 3 times. Each time they can't find anything wrong. Alignment is always dead on. struts appear good. the wheel rotates true. No indication of a bearing problem. CV doesn't seem to be a problem. So I took it to a an independent shop. They checked everything including the alignment and could not find an issue either. We have replaced all the tires and the left front is starting to wear again. Any ideas?