The immortal mantis has nothing to do with other mantis it is a separated case from any other mantis.
It is always an unique case.
It has appeared to me in almost all forms of mantis except harvesters and interceptors.
From V1 to Earth on all planets.
In several occasions I had to ask other players to kill it for me, as I have been in the middle of missions mostly when it has happened, and it didn't matter where I go it follows me and slowly takes my life away.
I can give examples what I remember,
V1 Avengers,
V4 Avengers and Marauders, also Marauder Commander
Terasa Parasite
Lustra Commander, Parasite, Barbarians
Prosperous Commanders, Rocketeers, Marauders
CC had Detonator
Techna Pyro
Baumar Avengers and Paralyzor
Mizar Avengers, Rocketeers, Commander
Mars Marauders, Paralyzor, Rocketeer, Assassin
Moon Detonator, Pyro, Marauder
Earth Assassin, Sniper
Some of these belongs to DS others not.
Particular is that it follows you out of their normal areas.
Most I have got when I have helped with Story Missions everywhere, or done whites myself at SOL.
I shoot at the mantis, the blaster just go round and round, and it is impossible to shoot even thou it uses up the e, same with Rockets and with Thermo.
I test shooting on other mantis works normal, then I return trying to shoot on immortal again, and nada.
Same goes on and on. In order to manage to finish up a Kill white mission what has to be done within marked area, I usually need to ask another player to kill that mantis or I do not manage to do properly my mission as it is timed and all I do is to flee and come back and flee again cause that immortal is really stuck on u with murder in it's eyes.
Edited to add new info:
Today I was grinding at Molikar rank 42 harvesters when I got an immortal rocketeer on my tail, as normally I couldn't shoot at it and I tried to pick up my cry from ground, and then I start shooting on harvs again.
What happened the immortal joined the tail mantises and I activated my perforator and aim-computer to shoot a rocket on the whole bunch, what happened the immortal was gone, I managed to kill it.
Now what I need is to reproduce the same situation, to have an immortal after me, and to use perf + aim and shoot a rocket on it, for to test if it can really be killed this way.
That would be a good thing, just taht I do not always have my storm in use, but my engineer, shock or tank as well time by time depending on the situation.
I will do a follow-up on this situation and edit this post if I get more info.