For record :
To prevent spell from stacking they have to use the same field "Spell.EffectGroup"
The targeted spellID are : 11644 and 11524, the first one have EffectGroup = 0, the second one have EffectGroup = 8, so they are treated like different effects.
I don't know if i'll be fixing this on storm anytime soon, it's data-oriented, it's not a side effect from my updates, and I have other spell review going on so I'll have more data to fix later