Hi FrantaB,
Welcome to Affinity Forums I'm not sure I understood your post properly so apologies if this doesn't address your issue. Affinity Designer (which I believe is what you are using since you mentioned CorelDraw) isn't able to perform boolean operations (Add, Subtract etc) between shapes and lines - only between shapes. So if you need to subtract a line from a shape you have to first convert it (the line) to a shape going to menu Layer > Expand Stroke with the line selected. You can then perform the subtract operation as usual.
Let me know if you still have issues or I haven't addressed it properly.
