There are two reasons why almond milk splits when it is added to coffee. Firstly, the heat of the coffee causes the protein present in the almond milk to fall apart. It's not a problem in dairy milk because the protein structure is more robust. In addition, the natural acidity of coffee can also cause the milk to split. To reduce the risk of this happening, always warm your almond milk before adding it to hot coffee. Also consider using barista blend almond milk as it is much less likely to separate.