Bug - Assistant gems on minions.

For some unknown reason, there is a certain interaction between some support gems in minions that causes an error in the spirit reserve. For example, my current build setup is as follows: I use skeleton warriors with the following gems (Last Breath, Infernal Legion, Fire Exposure: 120% cost multiplier, and Unstable Minions).

From what I can deduce with my limited experience, there seems to be an error—or at least that's what I think—related to the cost multiplier affecting spirit in certain situations, without clearly specifying whether it consumes extra spirit or not.

The error is as follows: I can summon a specific number of minions based on my spirit. However, as I progress through the game, instead of summoning, for example, 12 skeleton warriors, they automatically revive in different numbers—sometimes 10, then 11, then 9—without understanding why.