Partial protection can kill

I'm surprised to see that the AI does not cast individual protection spells when requested, if the mage already has some _partial_ protection.
I've tested a simple combo for early Pythium Serpent Cult : use hydras and serpent acolytes, but ask the acolytes to cast Resist Poison to avoid the poison of the hydras if they rout through my mages rearguard. But, no, the AI considers that a 50% poison resistance is enough, and the poor acolytes don't get immunity, and die if they are too close...
Same for Harbingers : don't try to script Lightning Resistance before Wraithful Sky, the angels will cast Mistform instead, and die very quickly to the lightnings...
Few mages have a 50% prot, but this is probably a serious problem if you try to use protection items like Dragon Helm, Fire Plate, Charcoal Shield, Frost Brand, Serpent Kryss, etc etc then try to give immunity to the holder.
Is this a bug ?
Is this because the protection will be greater than 100 % (50 + 100) ?
Wards work perfectly in the above conditions... but it gives 50 + 50 %. So it has nothing to see with the dom1 Thunderward/Wraithful Sky problem (we had to script WS before the ward, or the ward was overriden because opponent had no air mages).

Any thought ?

TIA