What you were expecting to happen, and what actually happened?
When the bombot dies it collects mana before its death e.g it casts spell goes to 0 then collects mana if it hits brown mana, while I don’t think it should work this way its text does read that way to explode gems first and destroy itself second in which it would still be alive and thus it collects mana. However to me it seems the bombot should be dead from cast and not collecting mana thus all and any mana from explosions should be given to other troops.
Maybe reworded and made to function as
“Destroy self, exploding 2 mana gems and deal X damage boosted by armor”
What are the steps to make it happen again?
Use bombot watch as it fills its mana before it destroys itself