Hi,
I don't' know if this is a bug, but there's a least a problem of consistency regarding the mage chill effects.
If a target is affected by Frostbolt and you cast Cone of Cold on him, it will remove the Frostbolt effect.
If a target is affected by Cone of Cold and you cast Frostbolt on him, the Frostbolt effect won't affect the target, even if it remains only 1 second of Cone of Cold effect.
Maybe it's a vanilla thing that better effects replaces and deny application of lower effects (CoC is 50% slow while Frostbolt is 40%), but the thing is : the other mage chill effects don't follow this rule. Ice Armor and Improved Blizzard can be applied on the same target, plus CoC or Frostbolt.
So, either CoC and Frostbolt should be able to affect the same target at the same time like the other chill effects, or no chill effect should be able to. Or there's a special rule for those 2 spells but mentioned nowhere.
Video : https://www.youtube.com/watch?v=HaJVXtyubEk