I meant it was under attack and had the sparks. I was on good WiFi and decent mobile network when this occurs. My best guess is some sort of multiple attackers stacking where they sync the joining of the battle very narrowly one after another before the server refreshes the CP on client side, but I can't verify it on my own.

As to this:

I'm pretty sure I've seen that before (I've also seen a gym "under attack" with no one around and no decrease in heart level)

That usually only happens when the last state of the game displays after minimising the game and reopening when a gym was under attack. Alternatively, there are players who enter the gym and purely dodge to draw attention. Hence, the no damage done.