Troubleshooting a soft pedal issue, came to find the left rear caliper piston doesn't advance. When you push on the pedal the piston comes out but as you let go of the pedal it goes right back in. Couldn't advance the piston by pumping the pedal (pads removed).

So without thinking much, I replaced the caliper. The new one does exactly the same thing. I've not seen this before. What could be causing it?

The master cylinder is new also. No ABS or any other codes or light. Any help appreciated.