[Question] Android randomly reboots, but doesn't ask for SIM pin code on reboot.

For the past few years I was starting to notice that every now and then my phone would just reboot for no reason. (Interval being like once or twice every 3 months, and no reason meaning no noticable reason (no software update, no freeze, no low memory, no extended use etc.)).
And this isn't even a normal reboot, because if I hold the power button and then reboot my phone, when it's done I am prompted to enter my SIM pin code, and then my device pin. However on these random reboots, the phone only asks for the device pin, never the sim.
When the random reboot happens, my phone vibrates as the screen fades to black, and soon after I see my service provider intro and the device and android specific intro, and then the phone asks for my device pin.
This was happening with my older phones, Xperia GO, Z3 Compact, and now it is also happening with XZ Premium. On the XZP, I have fingerprint detection on, however on boot I have set it to always ask for the PIN, and disregard FP detection on boot. That's why I know it rebooted, even if I don't notice it happening on the spot.

Anyone got an idea why this is happening? Always have latest available firmware installed (vanilla OTA android), and also latest app versions. (Also I don't usually shut down or reboot my phone, I leave it on "forever" most of the time and just charge it at night. Most recently I put it on charging and the phone rebooted when I plugged it in.)

What I want to know: why the reboots are happening, how to stop them, and how to force the system to ask for SIM pin when a random reboot happens (without it I lose an extra layer of security if this glitch happens).

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