It procd for me while using Inner Fire... right before it started a fight.
It has a decent proc rate tbh, 3-4 times or more / fight (didin't use it with proc addon yet)
How it is
Some spells like HOTs (Lifebloom, Rejuvenation, ...) or Buffs (Watershield, Thorns. ...) do not trigger the mana regeneration buff.
How it should be
The Memento of Tyrande gives you the chance to get the buff Bonus Mana Regen.
So any kind of spell you cast can trigger the mana regeneration buff.
Proof
wowhead
wowhead
wowhead
Some spells like HOTs (Lifebloom, Rejuvenation, ...) or Buffs (Watershield, Thorns. ...) do not trigger the mana regeneration buff.