Hymn of Hope does two distinct things when it ticks: 1. increase Maximum Mana by 15%, 2. increase Current Mana by 2%. When the first effect wears off, Maximum Mana changes back and Current Mana is left alone. Example:
I'm at 20000/1000000 mana, and I hit HoH. A tick hits me: effect 1 raises my max to 115000, effect 2 increases my current by 2% * 115000 = 3000. I'm at 23000/115000.
Another tick hits me: effect 1 does nothing (doesn't stack), effect 2 increases my current mana again by 3000 to 26000/115000.
Seconds later, effect 1 wears off and my max mana drops to normal. I end up at 26000/100000.