Unfortunately, this is one of MANY bugs in the game that most likely will not get fixed. I've pointed out to Deepolis on multiple occasions that to advertise a prize, in this case with a quest, and not provide the reward is deceitful (I'm not saying it was intentional or not). It was suggested that they simply change the verbiage to the quests to avoid this deceit - and by doing so, fix the quest system. Customer Support agreed with this suggestion but we still wait. Since this is known and some quests, like the BOSS quest, don't pay out as advertised (I don't want to accuse Deepolis with intentional deception) it has to be put in the same category as the Branka quest. The only difference is the player's aren't benefiting from these. Since it is in their favor (not the player's) don't expect a fix.
Do what the rest of us do: Put up with it. I would suggest to also send in tickets each time it happens - as a constant reminder that there is a bug that isn't being fixed. Apparently, they share these tickets with the staff.