Maybe the phone is lifting off of the charger and the charge stops/restarts? Not sure about the Pixel, but the iPhone screen turns on each time charging starts. Never had this issue, but maybe my phone is heavy enough to stay planted (I also don’t floor it often).
Yes the wireless charging pads are insufficiently bolstered for hard driving.
I sent my phone flying off once or twice in my M3P, though more from cornering than acceleration. Now I put my phone in the cupholders or other more secure location for really hard driving.
Mostly I just don't push the car quite that hard anymore after exploring its limits early on. Also I have a bigger, heavier phone now that stays put better (went from Pixel 3 to Pixel 6), well enough for twisty roads where there's no room for getting too aggressive.