It doesn’t happen consistently but I’ve fought him 5 times today and this happened twice. Basically during his 2nd phase, before he does the tripple-ground slam move, he seem to got stuck in a situation where it seems he would do an attack, but can not move his sword, I can keep attack him with impunity. During this time, his health drops normally, but his stance bar doesn’t move, resulting an easy kill.
Unfortunately I don’t have video of the bug yet.