Soft drop out materials are known for developing indentations from the QR end caps and thus the wheel will tend to settle into these "points of lowest energy". Have you tried a different QR skewer or a different wheel yet? Also the QRs with the fashionable exposed plastic bearing cams are notorious for their less clamping force compared to Enclosed metal on metal cams.

A loose drop out should be fairly easy to check for, not a bad thing to check but pretty uncommon, thankfully for the millions that ride bonded frames/forks. Andy