Using Summoner Spells then having less mana?

I can't exactly say the bug is related to using summoners but i've had multiple instances of having enough mana to use an ultimate, such as Jarvan's or Warwick's, flashing and then not having enough at my flashed to location. I've also had Ignite do this while trying to ignite then cast a spell.

I'm not sure exactly what it is or how it gets caused, because i've tested in customs and summoners don't seem to cost any mana ever, but I know its happening.

Has anyone else experienced anything like this or can explain it to me?