I have Mana Mirror in my inventory. It's linked to full mana pool with a Dominant spark. I also have Greater Band of Mana.

My mana bar is missing a third, and if I remove ring from my inventory/baubles my mana-powered items stop functioning. But if they're in use (say, I'm flying through the Flügel Tiara) mana mirror shows damage bar like if Band of Mana is pulling mana out of it (and the pool).

Lexica Botania entry says that "items will pull out of the pool itself", which seems to imply mirror wouldn't need an intermediate mana storage, and so does the missing chunk of mana bar.

Also, so far it's the second Mana Mirror (first was NEI-spawned) which broke - went blank with 1000 damage on the item, refuses to show the linked manapool, pings with no other display other than item bobbing up and down on shift-right-click, and when flying, does that rapidly like it's updating constantly. The manapool it was linked to is discharging quickly when it does that.
It also update-bobbs when inventory is changed - item picked or placed, including Baubles.
Oh, and when picked up with cursor or placed in non-player inventory, it lights up like full, but goes blank-and-empty again when placed back.

It's probably important to mention I'm storing it in TiCo Knapsack. It's light up like full while there, or while in any chest.
Band of Mana meanwhile behaves funnily - damage changes like it's discharging, but it's very hard to see due to mirror occupying part of manabar.

I'll have to look into it sometime, that item has had quite a few weird issues due to the way it works.