Well, Chasing got back to me with an answer, right away. Apparently, if you're submerged less than 0.3m, the lights will revert to 5% to prevent overheating. Good feature.
Have owned my mini S for a couple of months, and I'm generally quite pleased with it. However, lately the software is shutting down the LED lights to 5% after exactly 1 minute. Anyone else experiencing this?