Hymn of Hope buffs your mana pool by 20% for 8 seconds. Then it returns a % of your mana. The mana pool buff is intended to increase the amount of mana it returns. But at the end of the spell, however much it buffed your mana pool by, it then subtracts from your actual mana:

20k max mana unbuffed
24k max mana buffed

the spell gets me to 16k/24k mana
the buff leaves and I have 12k/20k mana

What is the point of it returning mana it is then going to subtract? A temporary mana boost isn't really helpful when I have no time to use the extra mana. This has to be a bug. When it shrinks your mana supply back to normal, it steals mana.