When a mob is chasing the player, the mob will randomly lose attention to the player for a short time and continue chasing. I was thinking it could be TPS lag, as my system is not very high-end. However, I noticed this only started happening with the release of 1.16. I observe that it's head moves back and forth, looking at the player, briefly gazing elsewhere, and then looking back at the player. I also find that it happens often in dark caves when zombies and/or creepers are present. I haven't been able to recreate the bug on video, but when I do I'll submit it.