you killed umbra with the bow, you didnt kill athron with it, to prevent transformation into Umbra.
you did not understand the difference, I killed the Arthron Umbra, it is the Pandoran closest to the Scylla, it did not die and then spawn, check the full video, min 7:00
after Tar Shadow you get this:
Exactly min 8:00 + some seconds, killed an burning Arthron Umbra with the Sniper and his crossbow, no Tar Shadow “prepare …”.
there is no Athron Umbra anymore… (1.7.3)… there are just Athrons, that have Tar Shadow ability… if you kill those with any weapon except flame, they summon the Umbra… you can kill Umbra with any weapon quite easily, because they do not have any armor…
But to prevent Umbra from spawning, you need to kill the Athron with fire. (but maybe LOTA weapons are exception… i dont know, i dont build them… I only use ordinary weapons, and if you kill burning athron with ordinary weapon, it will spawn Umbra)
They have changed a lot with the patches 1.7.1 to 1.7.3, maybe also some of these behaviours … who knows.
I see, every update or hotfix things are changing, those seem side effects instead of intended , but I don’t have any intel about it, devs are very silent lately
They “prepare …” for Steam release 
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Umbra Arthon’s don’t have 5k hit points either. 
Oh that Steam thing is sad news to me.
there are no more Athron Umbra… only normal Athrons… and those can sometimes envolve into crazy resistant bullet sponges with a lot of armor…
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