What triggers Always On Display's display of seconds?
Apparently, what I saw was not the Always On Display. I suppose the sequence of events was something like follows:


While the phone was in the pocket, power button got pressed
Due to hot weather the touchscreen was activated by the side of the pocket (wetted due to sweat), thus tapping OK in the warning dialog to "continue using the phone"
The same activation mechanism resulted in a tap on the clock area on the Lock Screen
Lock Screen presented its "extended clock" or whatever it's called, which shows seconds — on a very dimmed out background.


The dimmed background is likely what made me think it's completely black and assume that it's the Always On Display.

And indeed, this agrees with my observation that after several seconds the extended clock format changes back to the MM:SS — this is simply the display going to sleep, turning on the real AOD.