What is going on with Quinn's auto + passive out of ult sometimes not doing damage?

I've suspected this has been happening for a long time, but I just caught it in a scenario where it's 100% obvious that something is wrong.

https://i.imgur.com/77zcsdJ.gif

Qiyana's E + Q takes about ~30% of Akali's health, then her Duskblade + Electrocute + auto + Q takes another ~50%.

You can see I hit her and proc my Harrier mark, which should have dealt about 500 raw damage, but it actually seems to do virtually no damage. By comparison, Qiyana's E + Q should have had about 550 raw damage, yet takes 30% of her health.

Does anyone know exactly what is going on with this damage calculation? It only seems to happen when attacking out of ult.