I have used fleece for the front of a baby quilt and am working on one right now, and I decided to back it with fleece this time. Well, it is too bulky for free motion quilting. My Juki TL-98Q won't stitch on it, and I had a hard time too with doing some stitching on the front.
I have usually used flannel for the backing one ones I have made before, and never had a problem, but this time I couldn't even get the stitching on the front piece. Maybe the fleece is woven different, I don't know, tired everything with the machine and even tried my Viking, the thread kept breaking and the machines were skipping stitches. So who knows, every piece of fabric is different.
So this quilt I am tying off instead of quilting.
Good luck on your endeavors