Assize not returning MP?

I'm not sure if this is a bug or perhaps just a misunderstanding of the skill on my part, but Assize returns no MP a high percentage of the time I use it. I'm not sure if it's a bug, lag, or what have you. Here are some battle logs showing it.

When Assize works properly.
http://i.imgur.com/HVZ0XdM.jpg

When Assize works improperly.
http://i.imgur.com/obaQZpD.jpg

Some extra notes:

• I've only done this while running The Aery (I've not attempted it in other zones yet)
• It happens regardless of Cleric stance being on or off
• I've only tested it in battle (enemies and allies hit by it, no misses)
• I initially thought it was due to using Assize immediately after another spell, but even doing nothing for several seconds prior doesn't always prevent the issue