Both passive debuffs don't work with Grave of venom boss, the boss still attacks a random target even if you have let's say attack penalty on the harpy and either of fallen poison or sun pursuer.
#Astaion, Perseus passives bug
brisk lark
pearl thicket
Its not a bug, the mentioned skills aare special debuffs (different Border) and dont count as a debuff, unfortunately
Same as sime buffs have a different border too and dont count as "buffs"
brisk lark
The problem is that it's not stipulated anywhere. I wouldn't know otherwise.
bronze smelt
Fallen poison is so called 'gold' debuff and as such does not count toward 2 debuff limit on harpy.