Vaal temple boss K'aj Q'ura and ignite bug?

Ok so we ran a Random Vaal Temple yesterday. I play a very strong highly geared EA grand spectrum build.

We demolished the 2 bosses in seconds. http://pathofexile.gamepedia.com/Vaal_Temple_Map

Only the Cycloner (K'aj Q'ura) was left. Now I was fighting this guy for about 5 minutes before I died 2x. The reason for that is that he was not igniting.

Ok so after the fact and the analyzing of the video footage in slow motion I realized that he was in fact not ignited. Then I checked the wiki, and i googled the boss. In addition I checked the map mods carefully.

There is no mention anywhere on the internet thet that guy can not be ignited.

you can see in the video at 3 minute mark ~ that he gets ignited for 1 sec: https://youtu.be/4BTb89h8ukM?t=3m

So WTF am I missing...

As a side note: Everything else in the map ignites because I basically have 100% chance to ignite. And check this even the boss himself (K'aj Q'ura) ignited 1x for about 1 second. This seems very weird, so I'm seeking your knowledge dear Redditors.

Here's the pics:

http://imgur.com/kha3QFn
http://imgur.com/yehxNdb

Video (not edited!):
https://youtu.be/4BTb89h8ukM

Reddit discussion:
https://www.reddit.com/r/pathofexile/comments/5654ab/vaal_temple_boss_kaj_qura_and_ignite_bug/

PS:
If anyone interested in the build guide although this may not be the best highlight the build is strong af.
https://www.pathofexile.com/forum/view-thread/1733131

The mechanic you just met is somewhat similar to enrage. It is both on vessels and trio last member.

When only 1 vessel/trio member is left (doesn't matter which one) they gain immunity to elemental ailments. It has been like that since possibly the implementation of the Atziri map. If you had hovered over him, you'd have seen that he is in face Immune to ignite, however you are right that the fact that the enraged trio gets immunity is not mentioned anywhere.