Morlu Legionnaires lose their Firebird’s Ignite status when they start their death animation, which seems to happen while they still have about 5% of life left. Because they still have some life, they don’t die. The only way to kill them is to reapply Combustion and then Ignite them again.
This happens consistently with every instance of a ML in all areas and tile sets in which they appear. I haven’t noticed the behavior with any other enemy. I haven’t played the Firebird set before, so this is the first season in which I’ve observed this behavior. In order to reproduce this behavior it helps to be playing at least a fairly high GRift level so that your Ignite skill doesn’t kill the ML on its own.
I’m using Electrocute / Forked Lightning as my Ignite proc. Also using Focus / Restraint and Stricken, Trapped, and Zei gems.