I agree 100%. However, the devs don’t 
This has already been raised in another thread:
My argument is that if a monster is “paintballed”, then it is not captured, like in Pokémon, so that it is not with you in whatever habitat you now happen to be. It is only marked, and still roaming freely in whatever habitat it was in when it was marked. Therefore, it should contribute to “monsters slain in that habitat”. But unfortunately the devs don’t see it that way