When a mana pool is being emptied, a comparator measuring the amount of mana inside does not appear to update (however, it does when the mana pool is being filled).

Steps to reproduce:

1. Place a mana pool on the ground and a comparator facing away from it right next to it.
2. Fill enough mana into the mana pool to make the comparator output a reasonable redstone power level (maybe slap on a manaseer monocle).
3. Place another mana pool (or a mana cart) near the first mana pool and a means of moving the mana from the first mana pool to the second (so a mana spreader or a mana pump).
   • The contents of the first pool should now be drained into the second container.
4. Watch the comparator output.
   • Expected result: the redstone power level should drop until it hits zero once the pool is empty.
   • Actual result: the redstone power level stays the same while the pool runs empty.
     (Also see below.)

Checking the contents of the mana pool using a wand of the forest does not change the comparator's output, however, a block update will make the comparator output the correct level again (which may also be a way to fix this issue by having the pool cause an update at appropriate times).

This may be a vanilla issue, however, I was unable to reproduce this using items in a chest -- the comparator changed its output according to the addition or removal of item from the chest.

To cover one more base, here are the mods I have installed:

• Baubles-1.7.10-1.0.1.10
• Botania r1.7-237.jar
• GardenOfGlass
• NotEnoughItems-1.7.10-1.0.5.118-universal
• CodeChickenCore-1.7.10-1.0.7.47-universal
• InventoryTweaks-1.59-dev-152
• Waila-1.5.10_1.7.10
• Forge 10.13.4.1558-1.7.10

Hope this helps.