Did the mission succeed but you failed to take land, or did the spell fail completely? If the first, it's a bug, but if the second, it just means you didn't have 50 mages per mage tower.
Ok. But my second question, is how do I lose 40k mages agaisnt someone who has 0 mages? If you attack someone with 0 troops you don't lose anything, why is mage battles different? I tested out the same thing at Valhall and there you don't lose any mages cause your opponent doesn't have any.
here, you use a fixed (on average) percentage of your leaders per attack. The math is different from troop attacks. I believe one of the proposed suicide fixed involves making the math more like troop attacks, in which case you would lose 0 when attacking 0.