Android phone’s lockscreen PIN disappeared?

I own a ZTE Axon 7 (model 2017G) that I purchased secondhand from a reputable vendor in Germany ca. 3 years ago. The phone has been working well, and I updated the OS to Android 8.0. manually earlier this year using an official image from ZTE.

A severe case of butterfingers affected me yesterday (and I guess today), and I dropped the phone a couple of times (the phone has never been in use with out this very good TPU case from this company called Spigen). The last of these drops (today) broke the display. No physical damage, but half the screen is "gone"–it looks like the display pictured here in this iFixit thread but with multicolored dots instead of lines.

I wasn’t too worried about this (hopefully it’s just a loose cable), but then I noticed I was only swiping up to get past the lockscreen. Now here’s the thing: I’ve always used this phone with a 4-digit lockscreen PIN. I can confirm–that as recently as yesterday–my partner and I mentioned the PIN because my phone had died (battery ran too low), and they had to enter the PIN after restarting it; so this isn’t my imagination going wild.

As far as I can tell, nothing else has been affected. All the data still seems to be there, and nothing seems to have been "hacked" (I even briefly texted my partner with the broken screen to let them know that the screen is broken).

I went and checked in the settings, and the lockscreen PIN isn’t active there either as far as I can tell (so it doesn’t seem to be some kind of a glitch). I restarted the phone, and it asked for the PIN as it does normally to authorize the SIM. So it appears the lockscreen PIN has been disabled entirely, but it wasn’t me who disabled it.

How is this even possible? Can the phone being dropped at a weird angle disable the lockscreen PIN? (It sounds ridiculous just even typing that.) How can I check for signs of intrusion on the device (with the broken screen)?

Some "events"/facts that may be of relevance(?):

  • The one other question I have on this SE is about legitimate Google 2FA codes arriving from random numbers. This hadn’t happened in a while, but it happened again on 29.06.2020 (this Monday).
  • I always put my phone next to the bed during the night, and I did so last night. The phone was in another room this morning, however. Neither my partner nor I remember moving it. It is plausible that I moved the phone (I have a mild tendency to sleep walk), but I doubt I would have been able to disable the PIN in my sleep.
  • The phone’s storage is not encrypted.