Song of Wind vs. Iss' Elemental Protection

For some weird reason, Song of Wind and the Elemental Protection buff of Iss Enchanter don't stack. I have tested this a few times today.
It is reasonable that Song of Wind and Refreshing Sonata don't stack as they have the same +20 speed effect. But I cannot understand why Song of Wind doesn't stack with Elemental Protection either.

I wonder if it is intended by the developer or just another bug.

Well another weird thing is how cat queen buff doesn't stack with a certain sonata that doesn't provide crit dmg/crit rate. But stacks with warrior harmony which provides it.