Possible BUG - Character dead with MoM (Mind Over Matter)

Hi Dear GGG team,
My character DIED when my mana pool is not empty while i had Mind over Matter.

This happened during the fight with the T15 map boss-rathbreaker.

I had "Chaos Inoculation", so it is not caused by the unique mana flask.

I had two assumptions:
1, does the boss has culling strike effect?
2, does the character regenerate mana even after death?

Otherwise, i believe i should have 0 mana upon death, and there is a bug, please fix it.

It took me a lot of effort to level a lv93 character in hardcore. T_T

or can someone tell me what was the cause and i can learn.

best regards