Thralls not attacking enemies → Investigated, hard to reproduce

I have captured the following footage to help you see what we have going on. Some mobs will agro right away, others are delayed by 5-10 seconds, others rarely agro.

As for our trained thralls: The thralls in the video are not spawned in, they were captured, trained and placed. You’ll see how they rush to the spot of the fight, then just watch, never joining in. The same thing happens if you’re ever purged.

At the time time of this footage being taken, we had 19 players online.
Our server hasn’t ever been modded and it was a fresh map/server installation on the 10th May 2018.