Prism99

The white tufts coming through is called "bearding". Polyester thread would not cause this; it always involves the batting, sometimes in combination with the needle and/or the fabrics sandwiching the batting (especially the backing fabric). Warm and Natural is one of the few battings that has a right side and a wrong side. Bearding is more common when the batting is wrong-side up. However, bearding can occur with almost any batting.

Vintage wool battings bearded terribly; quilters had to encase it in cheesecloth before layering to help prevent bearding. Most modern battings, including wool battings, are treated during manufacture to reduce the chance of bearding. However, there are no guarantees and some weaves of backing fabric will permit bearding more than others. That is why it is a good idea to use black batting if the backing fabric is dark. There is no cure for bearding once it starts, which is why prevention is important.

Warm and Natural has a right side and wrong side because it is needlepunched through scrim. Needlepunching through scrim makes the batting ultra-stable so quilting lines can be quite far apart without risk of the batting tearing apart and balling up. You want the batting positioned so that your needle is punching through the quilt sandwich in the same direction that the batting was needlepunched. Battings that are not needlepunched through scrim do not have a right side and wrong side. It's only with Warm and Natural that you need to pay attention to which way the batting is positioned.