Einhar's Beastrite Modifier loss

I have found if you kill any other monster with the skill that uses Einhar's Beastrite after u have modifiers from a rare, it will overwrite the modifiers and you just lose them, I would think that it would only do that if the monster u killed was rare with modifiers to overwrite the old ones and refresh the timer.
This is especially annoying if you use a strike skill that will hit more than one unit or accidently miss and hit an unintended target.

Is this an actual bug or am I tripping?