Phone randomly reboots when waking up

REPRODUCIBILITY (% or how often): random, 0.5-1 times per week
BUILD ID: 3.3.0.16
HARDWARE (Jolla1, Tablet, XA2,…): Xperia 10
UI LANGUAGE: Slovenian
REGRESSION: (compared to previous public release: Yes, No, ?): same

DESCRIPTION:

Sometimes when I press the button to wake up my phone, it instead reboots (LED turns red, then the unlocked bootloader warning appears, etc.). Seems to happen completely randomly, but almost always when I try to wake the phone (a few days ago it happened overnight, while the phone was charging – when I picked it up in the morning, it was asking for my unlock code).
I had the same problem on Xperia X, but it was much more rare.

PRECONDITIONS:

Unknown

STEPS TO REPRODUCE:

No way to reliably reproduce – it’s random.

EXPECTED RESULT:

Phone wakes up

ACTUAL RESULT:

Phone reboots