Whipped Cream Separates While Piping

Are you using an insulated piping bag with a disposable plastic piping bag inside? Is the room hot? Is the whipped cream over whipped?

To solve the heat problem, try folding a dissolved gelatin sheet into the whipped cream.

If the cream is over whipped it will start to look a bit grainy, as the fat starts to glob together as the fat turns to butter.

I remember a pastry chef once explaining to me that if your pastry bag isn't totally clean and dry, any residual oils could "break" the cream, resulting in what you described. We would use DIY parchment paper bags for this very reason. And if the issue is indeed heat related just make up 2 bags and keep one in the refrigerator, switching back and forth between the two.