User blog comment:SirAston/URF and the LoL Engine - A To-Do List/@comment-5781707-20140402152520/@comment-5331854-20140402222225

That's more-or-less expected behaviour, though, so it's not something that can bug out the LoL Engine or give more insight into the specifics of the engine.

It's just the result of having lots of CDR, not something like Swain's Ravenous Flock or Kassadin's Rift Walk, which actually tells us that "checking against current mana" has more priority than "100% mana reduction".