Suggestion: If you have maximum HP, but not MP, upon completing an Adventure's Assignment quest, you get additional max MP (the same as mages) as a reward for completing the quest in the place of additional max HP.
The quest is not designed to be used to min max. It’s more of a quest to almost do passively on your journey to finishing a character and to reduce the need of HP washing. If you really want to get more MP out of it you could reverse wash via APR. Thanks for your feedback!