In my “case” it was the case that cause the problem 
I have an aftermarket flip-case from Xperia 10 II, so probably its magnet causing these phantom wakes for Xperia 10 III (with flap fully flipped open). As soon as I removed the case, 10 III stopped waking up.
Xperia 10 III doesn’t have hall sensor enabled (even in stock Android), probably because it’s flaky, and they may have disabled it altogether in later Android OTAs.
BTW that aftermarket magnet placement was off, thus useless even for 10 II if you wanted wake-on-flip to work.