[BUG] Morde W passive only works on cannon minion outside of Summonres Rift

Ive tested Mordekaiser's W on Twisted Treeline, ARAM, and Dominion and his passive only works on the cannon minions. This seems like a pretty big bug and has been going on for a while.

Repro Steps:

1. Get in lane with an ally as Mordekaiser
2. Kill minion
3. Notice no experience bonus
4. Kill cannon
5. Notice experience bonus