I make two rectangles in Affinity Designer. One is bigger and the other is smaller. I make center the smaller one on the bigger one. I make the smaller one on the top layer. I subtracted the smaller one from the bigger and there will an unfilled hole on the center of the bigger. Most time when I copy this object to the Fontcreator the unfilled area will be filled in Fontcreator.
Try reversing the contour direction on the inner contour, or use the Validation tool.
Validation works. Thanks!