Unintentional regeneration in Minecraft?

I and my friends recently noticed that I sometimes regenerate extremely quickly without using effects or hacks. So I easily win against my pro friend on Gomme in Pvp. He makes 3 times as many hits as I do and I still win with half Hp. So also on my Minehut server, on which we created a Pvp map. As I said, it happens rarely, and when I go to Gomme again the bug or something is gone, but not always at minehut. Is really annoying if you want to play a fair round of Pvp, so an explanation and solution would be really great!

If you play with 1.15 this is not a bug but normal.

If you want to play PvP, please use 1.8.9

But the others don't regenerate nearly as quickly.

As I said, they play with a different version