Actually if you compare the fastest Q3 laps, Max is only a few thousanths behind Piastri until T7, post which is where he lost all of the quarter of a second, a large chunk of it in S3.
My guess is that it's the super-inflated tyres (min pres 29F, 26/27R) , at high aero load (super low riding and at high speed as well), once they are upto 'peak temperature' after T6, causing a 'red shift' or 'blue shift' in tyre harmonics w.r.t typical numbers when pressures are normal, whereby the new frequencies now find some sort of resonance with the damping frequencies of the suspension, leading to unexpected vibrations, and cause the bouncing/porpoising, making the car lose critical grip in phases of corner entry/exit.
Remember, one of Max's comments was '
I feel the engine is jumping from it's mounts and moving on it's own, out of phase with the car'. That is a big clue
(it's wonderful that the team has a driver who is ultra-sensitive and feels everything happening in the car). I am almost convinced that this is exactly what happened with the Mclarens in Vegas (dangerously low static ride height, high speeds, and high tyre pressures) , just that because the track was bumpy, it led to premature skid wear.
"We're happy with P3, but we're even happier with the difference. We were about half a second behind all weekend, and now it's only two tenths." 
