This is an old thread but I can't find anything new about this so I hope others will be able to find this reply. I am using a work around for this problem. If I open a PDF of 100 pages Affinity designer will nicely layout those pages as 100 artboards. If I want to move page 4 (artboard 4) I can select page 4, zoom out, click and drag it down beside page 100 (artboard 100). If I export it to PDF that page (page 4) will ended up being page 1 in the PDF. Instead of dragging page 4, I select page 4 in the layers panel, hit control x, scroll to page 100 in the layers panel, select page 100, hit control v. It's still page 4 in the layers panel but now when I export it as a PDF it is page 100. If I want to move page 4 to page 30, I hit control x on page 4 in the layers panel, select page 31 in the layers panel, hit control v. Page 4 becomes page 30 when I export that to PDF. The layout becomes messed up but I will just open the PDF that I exported and everything is laid out nicely again with the pages reordered the way that I want them to be.