Assistance with Brake Squeal

Hello everyone--I've lurked for some time on this very helpful forum and have used a lot of the advice offered here, so thank you. My 2011 Prius developed a rear brake squeal after all brake pads and rotors had been replaced. The mechanic replaced the pads again, and, since the squeal did not go away and the car was also exhibiting signs of the 2011 actuator problem (pump running a lot, hydraulics getting weird), they next replaced the actuator and booster and bled the hydraulic system. The squeal persists. It only occurs when the car has been in use for 10-20 minutes, and it's only at the very end of breaking. The squealing gets increasingly louder the longer you drive the car. Any ideas about what is happening and what I should pursue next? Thank you so much for your help!