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Did you replace the pads and calliper yourself or did a garage do it? Why did you replace a calliper?
Long story short if a garage did the work I'd take it back to them to sort out.
If you did it yourself, is it possible you didn't wind the rear pistons back out fully? Were they easy to wind back? If not, they could be seized?
Otherwise you can try locking the rear pads by pulling up the handbrake several times to try and reset the pistons to the right position.
It could be a stretched cable or damaged housing, but I suspect not given the recent work to the rear brakes that you've described.
Mk6 Golf.
It's a manual handbrake - as in a lever you lift.
I guess I want to check before I autoadjust again that it's not symptomatic of something else like the cable, and if I autoajust it's not going to make it worse.