I have the same issue occurring. It doesn't matter if I modify the file or don't anything I export from Affinity Publisher or Designer as a wmf or emf file gets increased exponentially in size and won't open in the program being used.
I would ask the same thing as Kaseyh - is there a way to reduce the resolution when exporting to a wmf/emf file type?
Why does Affinity increase the size of the file exponentially? If I upload the file to see the metadata the file has a very small height/width but when I upload the one exported from Affinity it blows it out to 12 times the size on the height/width.
To add to this I also tried the same thing out of Inkscape and it works correctly. I don't understand what Affinity is doing in the export process. If I try to export to an emf it makes the file exponentially larger than it does doing it to a wmf. I've also tried the svg file types and can't open those once exported either. (Actually tried pdf, svg, emf, wmf, tiff, etc..... and none of them seem to work.)