I've never heard that! I've made a HS quilt and quilted it on a midarm (sitdown) with no issues with the bias pieces. I have to wonder if the long-arm quilter had it pulled too tight onto her machine and/or had a hard time keeping it straight? Or is it possible that the seams weren't a full 1/4 inch and pulled away? I'd probably ask the long-armer for more information on what she is meaning.
Wouldn't pretty much any quilt with HalfSqTris also have the same issue for that longarmer? Honestly I'd probably find another long-armer who can handle it better.