Ok, so, I cheked, it seems that it only happens when the enemy took 0 damage and has negative max hp. Realistically this should not happen, but i think it may be possible with someone like victor or viscous that can easily heal. My guess is that damage is stored as an integer, so it's x damage...
When i drain an enemy's health by killing them enough times to reduce their max hp below the range of a siphom proc, i can no longer kill that enemy. It seems it first applies siphon, then checks for damage, and i guess things that have 0 or negative hp just can't take damage.
Could it be that...