I have the same issue. I have an Affinity Designer 2 doc with multiple named artboards. I create a Publisher 2 doc and 3 picture frames. For each, I click "Replace Image" in the bar up top and select the Designer 2 doc in the Finder. In Publisher 2, I toggle open the picture frame and select the image inside so that I can access Properties in top bar. The Artboard dropdown there is bugged. The dropdown has these choices: Document, and then the names of the Artboards. When I select Artboard 1, I get Artboard 2. When I select Artboard 2 in another frame, nothing happens, but then I choose Document and I get the frame.
Anyway, it's a mess. Please fix!