Likely because Healing Rain uses a targetting circle and the /use command is cancelling it. Try having the Use on a nomod.
Code:
#showtooltip
/cast [mod:shift]Healing Rain; Frostbrand Weapon
/use [nomod:shift] 17
Just a question, considering healing rain and bloodlust are likely to be needed more than last minute weapon imbues - wouldn't it make more sense to have the imbues on the modifier?
To automate confirming the imbue change you could use:
Code:
#showtooltip
/cast [mod:shift]Healing Rain; Frostbrand Weapon
/stopmacro [mod:shift]
/use 17
/click StaticPopup1Button1