Sounds like the grip safety from the regular EZ wasn't installed correctly. Between all this and the holster thread, I would ditch the PC and get the standard EZ or move on all together. Did you ever shoot the Glock 43x?

I have the standard Ez in 9mm, thought about the PC edition and decided it didn't do anything for me except the flat trigger. But mine works flawlessly, so I wouldn't give up a good gun for the new PC. Too bad S&W wouldn't sell me the PC flat trigger separately.

Might want to have someone show you a good grip. I've never seen a slide lock/release from a S&W cause bite on the thumb unless it was an older alloy smith like the 39 with the huge slide release.