[Baron Von Zeppelin]

If you use a pry bar or big screwdriver to wedge some clearance in your pads to remove the caliper - the piston should try to protrude back out a little bit on its own from some of the back pressure still in the lines - if you leave it alone and observe.

If it doesn't do that , the piston is usually hanging in the bore.
Since yours are new they might not be properly finished all the way at the back of the bore.
Thinner pads will keep the piston more into the sweet spot - in a case like that.

If they are "new" Chinese , its likely that is a culprit.
If they are newly remanufactured GM calipers it would be less likely .

edit : the lip on the caliper piston seal aids in pulling the piston back when pressure lets off.
If the piston is hanging , it can't do that .