Oh dear, if it was regular Heat&Bond, it does not soften much. The good news is it is permanately fused and you might also put a layer of batt behind that appliqué and quilt top before stitching 1/8 in from the edge (trim away the batt outside the stitching line) of the appliqué for a trapunto effect. You then sandwich the quilt as usual and due to the extra “puff” behind the appliqué, it won’t matter if it is a little stiff.