[Quitti]

Probably buffing itself in other paths, like summon earthpower+invulnerability or something similar, which could bring it to 42 fatigue. Nothing to be afraid of. And as Squirreloid said, 12 spellcasting encumberance is 'crazy town' levels, which is not necessarily bad if you just want to throw that one big spell with the caster, but if you want to keep casting spells, reduce it somehow. Easiest would be to take off that black steel full plate from the caster or whatever it's wearing, put rainbow armor or any of the dragon scale mails on it, they're good for those squishy expensive casters (at least in CBM, I seem to recall them being somewhat useless in vanilla).

To the original issue, spellcasting AI has some renegade ideas about gems, they're like candy to little kids - it just can't keep it's hands away from consuming them. Sometimes it can do it if the scripting is done to do some full-scale evos and there's just few units against the army/caster and it decides to cast something else from another path. Like summon fire elemental. Or something.

What you should keep in mind: If you give gems to a mage, don't expect them to be there the next time you fight with them. He/She/It will use them at the earliest opportunity, even to reduce 4 fatigue per gem. You can go around this by giving a stealthy commander, such as a indep scout the gems for next few battles and transferring them at the turn next to big fights.