[2022] Last kill not counting as on task?

I've been killing abyssal lords for slayer tasks recently and noticed that abyssal flesh is a 100% drop on task but does not drop for the last kill for the assignment.

I wasn't sure if this is unique to abyssal lords due to them counting as multiple (5) kills per assignment so I tested and regardless of having 1,2,3,4 or 5 kills remaining abyssal flesh never drops on the last kill.

It seems like tertiary (perhaps all) drops are being rolled after kills are deducted from the assignment. So, the last kill of every assignment doesn't have "on-task" drop rates.

Does anyone know if this is specific to abyssal lords and are there any other slayer monsters with 100% drops on task we could test this with? Or has anyone gotten an "on task only" drop on their last kill?

*Edit: I have checked the wiki and did not find any slayer creatures that also have a 100% drop on task to test this with. There are a few "on task only" drops we could see if they ever drop on the last kill of a task but that seems kinda annoying.