had a caliper with a problematic piston before , very odd
kept sticking , worked it in and out and all was fine for a week or so , then the same again
I'd inspected it ,no rust etc
I replaced it with a known good used item
examined the dud unit with digital micrometer
the piston had gone "out of round"
which if you think about it ,can happen because the piston does get dragged in one direction by it's pad and calipers all the time during braking, not much but enough to cause the issue
If it was my car (or one i look after) I would buy a used replacement caliper(would be cheap and easy to get hold of) i
present for mot early , if there is an in-balance issue then you have the spare ready to go on
but you can't rule out other issues like a dud ,semi jammed/part blocked solenoid in the ABS module etc
IF all the slides / pads/ discs / fluid /wheel bearing etc is ok (including manufacturing faults so compare o/s and n/s for any slight size differences etc)
and you've checked the suspension components
both sides (it could be a duff component on on the non pulling side that makes the car pull to the problem side )
also, familiarize yourself with the RBT procedure as per the video and watch them do the test if possible
not that i'm saying your garage is one of the bad ones , it does sound like you have an issue
I just don't trust garages
I'm fortunate to have an MOT tester that is strict but fair
I've missed things in the past ,he has too , we are all human
(though Mrs Builder may argue that point about me
)