What triggers Always On Display's display of seconds?

I'm using a Samsung A320F/DS with AOD in "show always" mode. Android is 8.0.0, AOD has only a single page (without the player). Clock style is set up as

  22:59

TUE 3 JULY

  84%

But one day, when I extracted my phone from the pocket, I found it displaying time as 12:00:41, changing seconds every second. I didn't pay attention at first, and hid the phone back into the pocket. After some time, when I extracted it again to use, the seconds appeared to have gone.

But now I wonder: what triggered this display of seconds? How do I reproduce this? I've tried tapping everywhere, pressing sensor buttons at the bottom, holding them, pressing/holding menu button, volume buttons, but the phone didn't react.

EDIT: I've found that on the Lock Screen, I can get seconds by tapping the clock area. Then the screen dims a bit, and the clock begins to display seconds — until the screen turns off. I still wonder how I triggered this in the Always On Display.