Soft brake pedal

Sounds like you still have a bad master cylinder if there are no fluid leaks. The pedal dropping shows fluid transferring someplace. Master cylinder is the most likely. Not uncommon these days to get junk parts straight out of the box.
Try this test. With the engine off, pump the pedal a couple times to drain the vacuum booster. When the pedal gets hard, stomp it hard and hold it, does it still creep down?

Okay, you say it doesn't drop when you hold the pedal with the engine off? However if you hold steady pressure with the engine running it does slowly drop? If that is the case it has a pressure leak somewhere or it is bypassing in the master cylinder. A way to isolate where the issue is would be to disconnect the lines from the master cylinder and then plug the ports. If it still does it the master cylinder has leakage past the seals and pistons.
If it doesn't then it is downstream of the master cylinder and you can repeat that testing at the components. As I said above it's unfortunately common to get bad parts these days and getting worse.
Now if you are saying that it doesn't have pedal drop but is still spongy, you have air in the system somewhere. I've had vehicles that took 2-3 bleeding sessions to get all of the air out of the small ports and lines.