[Champion Skill] Mordekaiser W not giving assists when it does damage

Repro

1. Play Mordekaiser
2. Throw W on a teammate
3. Have teammate get kill or assist where your W on your teammate does damage to the enemy who got killed

I noticed this in ARAM. I threw my W onto my team's Kassadin, who blinked into melee range. I saw my W do damage to the enemy, around the Kassadin. The enemy was killed by Kassadin moments later, no assist was granted.

Happened (or... didn't... happen...) with throwing it on an ally who then had it do damage to the enemy, and they got an assist.