Sometimes a mob when hurt doesn't play its own hurt sound, but another mob's one. It happened while I was killing zombies in a dungeon, sometimes they would play like "skeleton hurt" on top of zombie hurt, even though there were no skeletons nearby. Same happened with zombie hurt + creeper hurt while there were no creepers.

This also is shown in the subtitles, "creeper hurts" or "skeleton hurts" when they play and shouldn't. I didn't have this issue in 1.15.2 or even 1.16, only 1.16.1.