Why does my mana gets deleted in boss fights? (BUG?)

Im a very geared arc lightning sorc with mind over matter eldrich battery chaos inoculation, but that is not the issue here. When I attack maps bosses my mana simple goes down to 0 in a few hits WHEN I ATTACK IT, not the otherway around. I have full energy shield. Its just impossible to get off spells in a rapid succession to delete the boss. Luckely im so strong the boss dies even so fast but I sometimes die if they get off a single attack on me cuz I have no mana and only can attack when my regen goes up.

When I attack rare mobs I have no problem with my mana and I can stand in hideout holding down arc lightning my mana does not even move. (I run with 40% reduce cost support gem too)

I mean seriously if this is a game boss mechanic its really shit design I just get tired of dying randomly when I have clearly overpowered gear.

Currently farming T15-T16 maps easily up until I face map bosses.

Can't get my last few levels to 100 due to this "bug", very frustrating to say the least.

I think I figured it out, could it be that the "Cast on Shock" spell I have also takes from my mana when triggered? I got Spark there with 192% energy+ and 60% chance to regain half its energy cost on trigger, which makes it trigger ALOT.

Does Cast on Shock trigger cost mana?

Yes. Whatever skill you have loaded into "Cast on Shock" will still cost its mana when it triggers.