Mordekaiser was made in a simpler time when there were a lot less moving parts in the game. Mordekaiser worked really well with few bugs back in earlier seasons. But as the game has grown more and more complex, the tech on his kit simply hasn't kept up. This is why he has so many bugs. Not because Mordekaiser himself is buggy, but because most of the new champions and mechanics introduced to the game function in ways that could never have been expected when he was released.
So, how does Riot fix this? They'd have to redo his code from the ground up most likely. If he were a champion like Lissandra or something who has a relatively modern kit and visuals/audio, then that might not be an issue. They could recode all of his stuff and then he'd be good as new.
But this is not the case. Morde is a walking garbage can. His kit doesn't make sense in the modern game, nor do his visuals or VO. Riot want to give him a full update. If they were to attempt to fix all of his bugs now, then when they get around to reworking him all of that bugfixing work is down the drain. The time that they would spend fixing these bugs is time they could spend coding completely new abilities for his rework.
I'm confident that Morde will be reworked in season 8, probably early on even, since he's one of if not the most problematic case and one of the most requested updates as far as I'm aware. So have patience.