I've had a1 report back since this guide where it fixed their Steamdeck and on investigating it turned out to originally be a 64Gb that's been upgraded to a larger size Nvme drive so looks like you're on to something with what you've said.
I'm sure Valve said that very same thing and probably for this very reason. I only found out it was originally a 64Gb deck 'cos I managed to find a steam page/link that included and linked all the decks info to my user account. For the past 3-4 days i've not had 1 issue with the deck either so...
I brought an older LED version Steamdeck today on a whim for £100 which had black screen issues but the touchscreen would work and it'd display using a dock but when not hooked to a dock it was black screen. The thing is, I'd tried near enough everything until I finally stumbled on the answer/s...