For whatever reason skeletons (Only entity i've tested atm) have horrible AI. I did a minor investigation and found that the goal target appears to be continuously set to null when a proper target is present. It appears to either do it completely randomly, or right after you hit a skeleton it'll set the goaltarget to null for a few seconds. In the video you can see a few skeletons just stand still, wonder off, or just completely disregard my existence. This is a major bug for any servers (Like my own) that is highly dependable on entity target functionality.

Video: https://www.youtube.com/watch?v=Vqcsl0uLleo

In the video you'll also notice i am not taking any damage. This is simply because i'm cancelling all damage events related to myself. I did this in order to capture a proper video without any knockback interaction or death. The bug is still present with or without any plugins, or health modifications. This video was conducted on my own localhost server with little to no exterior entities other then the ones in the video. The config files are also completely untouched, and i made sure to regenerate them aswell.