NPC sheet randomly rolling damage twice sometimes?

I'm testing stuff out for a Curse of Strahd campaign I'm setting up, and for some reason, the Specter sheet is acting weird. I removed the Life Drain description and put it into a separate attack, but kept the attack roll and damage. I have Auto Roll Damage & Crit disabled, and I have Advantage Toggle on. It rolls damage normally when the Specter has advantage or rolls normally, but for some reason, whenever I roll with disadvantage, it seems like it rolls twice (as if it'd crit when it hadn't) every other roll, if not more often. Here is a chat screenshot.

Whenever ANY of the dice rolled are a crit you'll see the crit damage added to the result (the + n ) bit. If the roll was normal or disadvantaged and the natural 20 isn't required just ignore the + damage and use the first number only. It's to streamline play for digital use.