Looks like you have inadvertently distorted the outer shape. You can create an object called a compound, this keeps all the shape editable so you can adjust them if they aren't quite right. You can make a compound by selecting 2 or more shapes or curves and using one of the geometry icons on the main toolbar above the workspace.
To make a compound...
Create the two shapes you want to use, so in this case a donut and a long rectangle that will stretch horizontally over the donut shape and overlap the edges of the donut shape. By example if the donut has a diameter of 10cm the rectangle should be approx 11cm. Make sure the shape or curve you want to remove is above the shape you want to subtract from.
Select both shapes, and while holding the alt key down click on the geometry subtract icon. You should now have a layer called compound and the two shapes nested underneath it.
giannis196 got a reaction from firstdefence in Affinity Design I have problem with curve after subtrack