Tags are empty. Other possible culprit is addition of Storage Drawers, no other changes were made since it was generating non-tagged redstone.

Additionally, I'm utilizing "redropping", aka taking redstone that was just picked up and dropping it back into the pool for faster conjuring. Which seems to glitch, as, for example, I've just dropped 27 tagged redstone in, and got 54 normal redstone back... plus 7 redstone, tagged, on an almost run-out pool.

Trying to convert with not enough mana is practically guaranteed to return tagged item. E.g. I've dropped 64 redstone in a pool with enough mana to convert 13, and got 26 normal redstone back and 51 tagged redstone.

Actually, it's worse. Dropping anything whatsoever in the manapool that cannot be worked by said manapool (for example, a lever) returns an empty-tagged itemstack, provided it didn't have NBT before. Or if it didn't have time to work on it, as per "dropping too much redstone/iron" example. Item on top of manapool, didn't get processed and doesn't have NBT already = empty tag.