I tried to replace the negative energy imbue on my short sword with frost, but instead of replacing it, the frost got added to it and I only have 1 level of spellsword.
It looks like the damage boost from swashbucklers insightful strike interferes somehow, since when that bonus wasn't there, the effect got properly replaced, but when it was there, the imbue was added instead.
Steps to reproduce the Bug:
Cast negative energy ray on weapon
Reequip weapons so the insightful strike bonus gets added again, if it isn't there
Cast another spell (I used ray of frost) on the weapon
Expected Result:
The 2nd spell replaces the current imbued element
Actual Result:
The 2nd element got added so that it had both negative and frost elements imbued
Screenshot(s)

Additional Information/Notes