Brakes squeal when car is in reverse

Hello,
there are a variety of reason brakes make that noise.
It may be as simple as dissabling and cleaning the moving parts and lubricating the pad contact sures and hardware.

If it is just the first stop or so of the day in the morning, then is may be a little moisture being heated and dissappated.

There may be a glazing on the brake pad surface or hard spots in it.

There may a ridge that the pad is touching or the brake sensor.

IF the pad shifts a slight bit it will make a different contact to it's surroundings versus going forward and stopping.