I don’t know if it is an Android 10 issue, my phone even reboots when it’s not switched on. It happens when you plug in the power and when the phone is off. It briefly flashes the big battery indicator on the screen and switches off. But immediately switches on again, flashing the batt. indicator, and. switches off again. Etc. I tried everything, including full factory reset and of course wifi off, no sim cards, no sd card (didn’t use one 
) It almost feels like a hardware issue, as it always happens when you plug in or remove the usb cable (and the ‘rebooting’ when not switched on, so it happens before any of the systems are technically on)
Edit: like this one: Random reboots every few minutes, cannot use my phone
And I did not have any significant updates recently. The Android 10 upgrade was more than a month ago? Don’t even remember.
edit 2: I never use wifi as I have an unlimited T-mobile account. It is hardly ever switched on, only on rare occasions.