Adding onto this;
Duo'ed with him for two games after this post was first made, and went back to check the replay of our last game - At least 2 seconds or so after casting his ultimate, his Q (among the aforementioned Aatrox's Q) outright did no damage at the centre of his mace's impact zone. I have the replay files and highlights to confirm.
However, we retested this in a custom game and it seemed to be working as intended, so we're unclear of the cause at present.
UPDATE: I've complied a Playlist of instances over 2 games where this bug occurs. However, I'll also put the interactions in text below.
- Clip 1: Mordekaiser uses his Ultimate on Riven. When Mordekaiser uses his Q, he hits the minion wave and Karthus while still in his ultimate (as detailed by the fact that Riven takes no damage despite being the Ultimate target, but then takes damage from his ensuing auto attack.)
- Clip 2: Mordekaiser uses his Ultimate on Riven. When Mordekaiser uses his Q, Mordekaiser hits Riven but deals no damage to either her or her shield.
- Clip 3: Mordekaiser uses his Ultimate on Viktor. When Mordekaiser uses his Q, Mordekaiser hits Viktor but only deals damage to the minion wave.
- Clip 4: Mordekaiser uses his Ultimate on Aatrox. When both Mordekaiser and Aatrox use their Q's, Mordekaiser's Q fails to damage Aatrox, while Aatrox's (2nd) Q damages and CC's the minion wave only, despite Mordekaiser being in Aatrox's Q's sweet spot.
- Clip 5: Mordekaiser uses his Ultimate on Viktor. When Mordekaiser uses his Q, Mordekaiser hits Viktor but deals no damage to him. Sidenote on this clip; I'm unclear if the Q in question applied Grievous Wounds, as while looking back on the replay the timer doesn't reset properly after his ultimate is applied.
From these 5 clips, it seems as though Mordekaiser (and possibly other champions) have certain abilities occur outside of his Ultimate when certain conditions are met, as we attempted to replicate it with no results. I haven't been able to find what triggers it personally while scouring the replays, but I still have the full files handy if anyone wishes to take a look.