Revisiting this after taking a couple weeks off from exploration. I think I may have some better insight now on exactly what is occurring. I think the gist of the problem is that overkill damage is being reduced to exact number, rather than actually applying all damage at damage step and simply displaying a reduced number.
- Cast bombot
- Because bombot must die, damage must be calculated at this point
- Damage is calculated for life drain troop and improperly reduced to exact armor + hp
- Bombot dies
- Life drain triggers, granting +4 life
- Improperly reduced damage applied; life drain troop survives because of the bonus
Of course, none of this still explains why single skull point of skull damage will disrupt the issue.