disc brake squeal

[eddief]

i am relatively new to disc brakes, these on my 2019 Roubaix, these being Ultegra. the pads were replaced by LBS about 300 miles ago and they were perfectly fine for all that time till now. high end shop so i think the pads were bedded properly. they don't make noise while braking and still having a few rpms, but just before the final stop and as i release the levers at the stop, they squeal like crazy.

i cleaned rotors with alcohol and sanded the front pads but no improvement. should i assume the pads somehow got contaminated and are not fixable?