Rear brake dragging after brake pad replacement

Hello guys

I have replaced the brake pads on both front/rear and also flushed the brake fluid. Below are what I did initially

1. Install new pads, grease moving parts including caliper pins.
2. press brake pedal several times to seat the piston
3. Flush brake fluid(which requires parking brake to be engaged)

Unfortunately, afterwards, I noticed that rear brake was dragging and was not rotating quite freely, especially passenger side. So then I did below.

1. Made sure X on the caliper piston is aligned properly as mentioned in many threads. Pushed the piston back in.
2. Caliper pins are greased and moves freely.
3. press brake pedal to seat the piston but not engage parking brake after.
4. Went for a drive.

What I found is that, after I did above, I only used the brake but did not engage parking brake. Drove around for 15-20 min using the brake ample time and when I got back, rear wheels rotate relatively freely similar to what I have seen on youtube.

So now, my thinking is that when parking brake is engaged then released, caliper piston is not retracting as it should. I'm not quite sure why this is happening because when I pressed the caliper piston for the new pads install, it rotated and retracted fine, and it does not seem to be seized.

Has anyone experienced anything like this? What forces the caliper piston to retract after parking brake is engaged? Is there anything that needs lubing to make this work better? any input is apprecated!!!