Bobocop said:We still don't know how precisely, whether he was like that all along and hiding it with the force, or if his dark force energy turned back on him caused it, or if the heat of the lightning melted his face... but it certainly was bright and colorful, and that's what matters in the end.
Seeing as Luke got a full blast unreflected for several minutes longer than Palpatine did reflected it seems that having his mask pulled down is the only explanation that makes sense.