Sprites that uses images from different directory levels don't work

Assume we define the sprites as follow:
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/assets/css/library/mystyle1.css (...background-image: url(../../img/icon1.png); /** sprite-ref: iconSprite; */...)
/assets/css/mystyle2.css (...background-image: url(../img/icon2.png); /** sprite-ref: iconSprite; */...)
/assets/img/icon1.png
/assets/img/icon2.png

/** sprite: iconSprite; sprite-image: url('../../img/bg_sprite_icons.png'); sprite-layout: vertical */

The resulting sprites and css will be:
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/assets/css/library/mystyle1.css (...background-image: url('../../img/bg_sprite_icons.png');...)
/assets/css/mystyle2.css (...background-image: url('../../img/bg_sprite_icons.png');...)

Notice that mystyle2.css is referring to the generated sprites ("../../img/bg_sprite_icons.png") now, but the correct path should be ("../img/bg_sprite_icons.png") instead.