I have noticed that copying images that are contained in Microsoft Powerpoint presentations or Microsoft Word documents and pasting them into Affinity Designer significantly reduced the quality of the image. The best way I found around this (after messing with a bunch of settings such as color formats, color profiles, DPI, pixels, Millimeters etc...) was simply to use the snipping tool of the image in powerpoint and paste that into Affinity designer. This did not reduce the image quality. But this seems to be a rather silly thing to work around.