Intellect not affecting blind and fear?
 

My problem:

I raise my intellect above 15, and duration of blind and fear (maybe few other spells) stays the same: 2 rounds.

Other spells, like slow, haste etc work fine: after raising intellect to 15 duration increases from 2 to 3 rounds.

Is it a bug?

Nope its not a bug.

The bugs are errors in the in-game and in-manual descriptions of the effects of higher intelligence.