BUG - Necrovigil on Undying Workers - Docks Cruel

Hello,

I wasn't going to post this because I thought maybe it was a one off bug, but I've noticed now a couple of times while I've been running Docks that the Undying Workers blue mobs that have Necrovigil DO NOT spawn the necro vigil when they are killed.

Not sure why, I don't mind for now as it makes them easier to kill but I assume it isn't intended :)

Thanks!

Maybe it is their name? Undying? 2 positives make a negative?