Mordekaiser Q Bug (no dammage)

The Q at 0:05 is a Phantom Dancer shield, I think.

But the real mystery is the Q at 0:14. Phantom Dancer procs at the same time as Nullifying Orb - for damage which would reduce you below 30 % hp - so that's out of the question.

We see the Mordekaiser ult going on CD just as the battle starts, so I'm assuming that this battle all happens in Mordekaiser's realm. Now, the fight is all in slow-mo, and Mord's ult has a duration of 7 seconds, which would then be extended to 14 seconds in slow-mo. In this case, what floo is writing is probably correct. The ult likely ends just as the Graves's and Mord's Q's are being cast, which nullifies their damage because the abilities are treated as if they're in Mord's realm while the champions themselves have been kicked out of the realm.

The TL;DR - it's likely not a bug. Just very bad luck.

So the first one seems to be some kind of shield. Maybe he had nullifying orb? The second one is morde casting his Q while the ult disappears. Nothing in cast animation deals damage if you switch between realms. Graves Q did not deal damage either.