Thanks for your answers. So if I'm understanding correctly, Affinity converts all documents to its own native format and then reconverts them for export, rather than editing and resaving in the document's original format?
That would explain why line objects get converted to SVG curves and paths as well.
GarryP, I have tried your help suggestion of placing, rather than opening, my files. Unfortunately I got the same result.
Inkscape exports all the metadata correctly, although it does add its own data too, but isn't nearly as nice to use, and doesn't integrate with publisher and photo as nicely. Illustrator does the job well keeping svg in svg and saving something nearly identical to the original file, but costs too much.
Palatino, yours would be a workable solution, but the custom attributes are spread throughout each document, not just in the header, and it's really not practical to edit every document by hand; that's what computers are for
Just when I thought I'd found the perfect suite of tools, this one tiny detail gets in the way. But I appreciate your help!