For what i found out is that when you  engage in combat and try do sex actions against the invader, they  can for some reason ignore your turn and keep attacking you. This only happens if you are nearing climax with the invader and do the sex that they are for some reason able to attack regardless of their stamina and mana being at 0.

I think its a bug in how the AI tries to recover so that they can fight, but since it dont have mana or stamina, its like the same if you deplete all our own stamina and mana that you have to skip turns, but in the invaders case it does not care and can attack anyways.

Noted i was confused when it happend because i was instantly spammed by them when they were already stunned.
So i was only going by what i was doing before  it started to happen, but its possible that it might be from how the ai tries to recover when its in 0 mana and stamina.