Slow on hit effects not stacking properly

[Benly]

My current character is a Brawler with a helm and boots that apply "slows global speed by 30% on melee hit" and gloves that apply "slows global speed by 30% on weapon hit". If both the hat and boots are worn, they stack and the Slow debuff on enemies reports -60% global speed. However, the gloves do not stack with either. If neither hat nor boots are worn, the gloves properly apply -25% global speed (again, according to the slow debuff on the enemy). It seems like a bug to have some slow on hit sources stacking and others not stacking.