[Progression] Fixed a bug where some Daily Quests still mentioned being able to be completed in Mercenaries.
(From 30.6 Patch Notes)
Dunno what’s that supposed to mean (does anybody?), but it might be related, including their general ‘policy’ (if one exists) towards quests’ completion in Mercs.
(UPD) Oh, nevermind, just dug this up:
https://www.reddit.com/r/hearthstone/comments/1f4o7eh/comment/llbz3c0/