Hello, that was a skill issue from my side >.< if it brings you any solace, the softlock only triggers when that attack would bring you down to zero HP, and without the softlock you'd be forced to restart anyway, albeit a bit more gracefully :p
Thank you for playing!^^
EDIT: owing to new information I have realised that this can also occur at an abnormally consistent rate even when the blow is not a killing blow, only at the Death fight. I truly apologise for this.