This often happens when too much liquid is added, but there are ways to fix it.
This is one way to fix broken frosting:
Scoop all the frosting into another bowl. Wash and completely dry out your mixer bowl. Add a few spoonfuls of butter to the mixer and whip on high speed (if desired, also add a little confectioner's sugar to balance it out--but the butter is the only part you really need). Then, add back the "bad" frosting, one spoonful at a time, to the mixer. Don't add the next spoonful until the one before it is completely incorporated. Stop short of adding the last few spoonfuls. This should turn all your frosting back to good, smooth, fluffy frosting in no time.
I haven't added milk to my buttercream in the past so I'm not 100% certain what might be going wrong, but two things come to mind that could be going wrong. Is it possible that the buttercream and the milk are at super different temperatures and the milk is shocking the buttercream a bit? That springs to mind as a possibility. Or try adding the milk extremely slowly while you whip (barely a trickle) - could you be adding too much milk?
Are you adding cold milk? Try nuking it for a few seconds to bring it closer to room temp and the temp of your BC. And are you starting with soft, room temp butter?
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This is one way to fix broken frosting:
Scoop all the frosting into another bowl. Wash and completely dry out your mixer bowl. Add a few spoonfuls of butter to the mixer and whip on high speed (if desired, also add a little confectioner's sugar to balance it out--but the butter is the only part you really need). Then, add back the "bad" frosting, one spoonful at a time, to the mixer. Don't add the next spoonful until the one before it is completely incorporated. Stop short of adding the last few spoonfuls. This should turn all your frosting back to good, smooth, fluffy frosting in no time.