this happened to me earlier today.
was going for a suicide headbutt on second attempt to finish off the opponent (had stockpile + 3 red/yellow ap for first turn headbutt), i had juggernaut and another character up (ofc i had a full party but the third char was pretty much dead with less than 100 hp). jugg has about 150 hp. i use headbutt, jugg’s health drains and everything, he gets downed, but the enemy ended up taking no damage.
this has only happened to me once, btw. the map was the jungle one if i remember correctly.