PS.GG was reported at 495 mph; that equates to M.645 at 5500ft at an assumed *T of 20*C; M.75 would be 579 mph in the same conditions.
A little slow for a strong shock wave to form unless he was pulling significantly more than normal turn g. But again, add in the Mach buffet during testing, and the tab rod or attachments could have been fatigued.
Shock strength and location are dependent on Mach number and CL. You can see what Mach buffet looks like from the tufts and the wake rake/camera vibrating. Mach buffet is just a stall, but at substantially higher q's so the loads are higher.