During a jungle setting patrol mission vs Chaos my melee bar disappeared while fighting the first Lord of Skulls.
It happened after I missed my first swing (either too early or too late I forgot which.) On the second swing the melee bar disappears until I tap on the screen, but since I can't time it I miss again usually resulting in my death. This happened on two attempts at the mission and the third time I had to resort to shooting down the boss instead of engaging melee.
Scott Chen