Is there some sort of rare spawn in the DLC?

Noticed after having completed everything in the DLC I’m still showing an Undiscovered challenge under Hyperion enemy kills. Curious if there’s something I missed or perhaps it’s TVHM only.

its the debt collector who is a rare spawn but he only spawns after you do the side mission sisterly love.

I’ll have to go spawn him again already got he trophy for it but maybe it’s bugged in the sense you get the trophy but not a kill if it’s killed from enemies.

Went and killed him again twice and can confirm that’s not it.