Why does the bell on the boss take more mana than it should?

Hi. My situation. I always use tempest flurry, including for movement, and my mana always has time to regenerate, so I always have enough. But when I use the bell in a boss fight, my mana consumption increases 10 times. I don't understand why this happens. I don't use cast on shock or anything like that to work on bosses. I did an experiment, and put the bell without a boss and my mana consumption was normal. But as soon as I attack the boss, mana consumption increases. The skill tells me that the bell costs 120 mana. But this is obviously not true. The game does not simply subtract these 120. It increases mana consumption several times. Can anyone explain how mana consumption occurs using the bell?

I recorded a video example
https://www.veed.io/view/000f80b7-e45f-4fb4-b51e-7de66217edd2?panel=share