[Gameplay] Mordekaiser Ult Bug?

Mordekaiser's passive, Iron Man, from the wiki: "Mordekaiser converts 35% of the damage dealt from his abilities into a temporary absorption shield (half as effective against minions and monsters)."

However, Children of the Grave does not convert any damage into his shield. Maybe this is intentional, maybe not, I just thought it was worth mentioning.

The best way to reproduce this bug is to:

1. Play Mordekaiser in a 1v1 custom on Summoner's RIft
2. At level 6, Ult the opponent champion, but do nothing else. No attacking, and make sure they do not attack you.
3. Watch as the passive bar does not fill at all.