I have what sounds like the same issue. In my case it is because the car is actually moving slowly backwards when on an incline. Even if leave in gear, it will do it a little before settling.
I found that the handbrake on one side wasn’t working, meaning that the other side was doing all the work.
Easy way to check is to jack up the car and have both wheels in the air and handbrake off. Pull up the handbrake one click at a time and check to make sure each wheel is tightening evenly. In my case, one was tightening up and the other barely changing. I have a new caliper on the way.