Bugged Spell?
Apparently there's an ongoing bug regarding Mordekaiser. Weird thing is I see no one reporting that even though it has been persistent for some time, every game I have played since the previous patch. I suppose it could be some local issue. Anyway, I suggest testing if others can't aim his W in Howling Abyss (Summoner's Rift is all fine) game modes too. I had couple of ARAMs, tried custom too. Every single time the W targets the closest champion, not the one you aim at.