I'm trying to use Mesh > Boolean > Union to cut a hatchway between two spaces and the first time it worked fine, deleted the ends of the doorway shape and kept the middle that forms the "door jam" between the two walls. But after the first time I used it, every time now it deletes the walls and middle section instead, keeping the ends.

I've tried changing the order that I select the two objects in expecting it would reverse the results but it had no effect???

Is there any way to force the function to reverse which parts it's deleting and which it's keeping?

Could it possibly be related to the direction of the normals?

Any help would be appreciated, thanks!