Mordekaiser's W refuses to target

I have noticed this bug twice in ARAM, but testing against bots on Summoner's Rift failed to replicate it.

When casting W (harvester of sorrows) it will refuse to target the ally I tell it to, instead targeting the closest allied champion regardless of direction or proximity to where I clicked, similar to if it was cast on myself. It will refuse to target any minions (I used to use it on a minion wave to get into lane slightly faster) and if no champions are nearby it will cast, but only on myself.