M3 MBP stuck shift key persists even after Apple replaced the keyboard

I have a 2023 M3 MBP. I started having an issue where it looked like one of the shift keys was physically stuck on (keyboard viewer showed shift being continuously enabled). Took the machine into a local Apple Store where they removed both shift keys, cleaned around the switches, and the issue persisted, so the machine was taken for a full keyboard replacement. Quite expensive as I didn't have the machine on Apple Care.

I get the machine back after repair where I was told the entire "top unit" including keyboard was replaced, and immediately after leaving the store, the issue is back. It seems that opening and closing the screen momentarily stops the issue, which is why I was able to log in at the store upon pickup, but within a few seconds, shift again becomes stuck on.

It can't be the keyboard because it was fully replaced. I just did a full OS re-install, and still have the problem, so it can't be an OS glitch.

Any ideas what the underlying issue could be? A USB port vault? I'm quite miffed that they replaced the keyboard at large expense (~$900 CAD), but that it wasn't the underlying problem at all. Waiting to get the machine examined again, but there's a long wait for a Genious bar appointment due to iPhone 17 launch.