I had the exact same happen to me on my last boat. I sanded off the varnish and sanded a tiny bit into the epoxy. My conclusion - wet sanding cloudy water not fully washed off. I wet sanded the epoxy before varnixhing and I fully believe it was cloudy epoxy dust water not fully washed clean prior to varnishing. By the way , I only sanded through the varnish in the areas that needed it not the entire varnish job.