There’s been this bug since the task system’s rework, where mercenary tasks can be completed in PvP even if the merc in question is not in the party which fights in the pit, but is assigned to a party active in a bounty [1]. Didn’t bother to make screenshots (perhaps I should have), but I’ve tested it now (as of 25.0.3) with Chromie — it’s still present.
[1] https://www.youtube.com/watch?v=UklCSkkHKtw