I was attacking a dwarf city, in the last minute of the siege a singular slayer pirate is capturing the gate.

I initiate attack orders with Blood shrines and bloodletters. Both units seemed equally "confused" when attacking and pathing to the slayer pirate.  The units both seemed to correctly path to the Slayer Pirates location and initially activate their charge animations but both missed their charges and then both units began to idle. 

I started spamming attack orders with just bloodletters which cover more surface area and should hit the Slayer Pirate more easily. And while they would initially path and move. They would then stop and idle. 

The slayer pirate was moving around erratically and unusually, moving through the bloodletter formation and attacking them, the bloodletters were still idling and not attacking the fast moving slayer pirate. It was very Benny Hill like.

Eventually I sent my 3 Heralds of Khorne lords to kill the Slayer Pirate, and they faced the same issue. Eventually, one of the Heralds initiated a sync kill on the slayer pirate and I won.