This has been reported several times before:
It’s very likely that you are noticing a confusing animation. At all game speeds, upon gem matches, the troop’s mana number is updated instantly, but the swooping animation flying toward the mana number takes longer to display. When the animated effect reaches the total number, the number doesn’t change because it already changed instantly upon the actual matching of the gems. This is more noticeable at certain animation speeds.
If this is not the case, please try to capture some video of the bug happening. The devs would genuinely love to see it happen, because it gets reported somewhat frequently.