I tested Copy Merged by copying content on 2 of 3 layers of an artboard. I made a new document that shows me the clipboard dimensions in the New Document dialog. It is working for me. I'm on Windows 10 and using Photoshop 21.2.3. I tried a few different approaches such as ungrouping the artboard, using multiple artboards, selecting multiple layers, not selecting layers, and using the lasso tool and polygonal lasso tool instead of the rectangular or ellliptical marquee, but so far they are all the copying the selected dimensions and not the canvas size. Could you describe the steps you took and the show the set up of your layers when Copy Merge didn't work?