I tested this with 3 different forms, and Resolve of the mandrill seems to cap out at 10 mana per tick, even for forms that already cost 10 mana. For my form that normally cost 15 mana per tick, that were only reduced to 10 mana per tick.

I could have sworn that the mana per tick was halved before the recent changes to mana costs for major/minor forms, only costing 7 mana per tick. Can somebody confirm if this got broken, or if resolve of the mandrill has no effect on mana cost after a certain level of mastery?