Mecha set bonus.

[Yasha-7HS]

It just doesn't work. I've had a marked target about six times in four hours, testing public games, hosted public games, not hosted public games, and solo games. The Kubrow doesn't even mark anyone when no one's being killed and it's just IT doing damage. I have a video of me radial blinding everything for 5 minutes while it attacks (AND DOESN'T EVEN HIT THE ENEMY ITS ATTACKING 75% OF THE TIME BY THE WAY, WHEN IT'S GOD DAMN BLIND AND STANDING STILL) alone and marks nothing at all in Sedna, Hydron and Fortuna. I've tried taking off precepts, putting on precepts I didn't have on prior, mixing precepts, reordering precepts, reordering the entire mod loadout, re ordering the mod loadout on my own warframe since there are mecha mods for the warframe that might for some strange reason act as a precept.

 

What finally worked was being in a solo match, leaving my warframe through my operator and getting back into it after about 1-2 seconds. I'm playing Excalibur Umbra. He also apparently can't gain the Mecha Pulse buff if he kills it while not piloted, so there's that.