A frosting splits when there's a large variation of temperature in the bowl. It can be that your almond milk or butter are too cold or you added more powdered sugar-free erythritol. Erythritol decreases the temperature of the batter. You can fix a split frosting by adding warmer ingredients in the mix, more room temperature almond milk for example. If it splits because the mixture is too hot, pop the bowl in the fridge.