Kovid, Until now I have been doing cut and paste from the opf files into exiftool commands - not a pretty sight. With this unexpected new feature the only way I've found to get the metadata that I have in the database into a PDF, is to convert the existing PDF to new PDF in the latest release.
I've tried various Save Book options and all I get is the what's in the book folder, same file size. checksums, and file dates. In case its relevant I do not retain ORIGINAL_xxxx files in the book folders.
When I do the conversion then - if the input PDF has no cover (most of mine don't they are reports, not books) then the cover.jpg file in the book folder, that I created for my convenience when using calibre, is inserted at the front of the book. I appreciate a cover is not metadata per se, but I get it as a by-product of doing a conversion, the sole purpose of which is to put the metadata into the book .
Psst - given my druther's I'd rather have a Polish PDF that just pops the XMP into the PDF and leaves everything else alone. Because as well as putting an unwanted cover on the front, the output of the conversion is far from satisfactory. The centred title comes out left justified, every line looks like a new paragraph so the document is more or less unreadable etc - yes I can probably fix that by tweaking the conversion settings, but my objective is not to convert the book, my objective is to update the embedded metadata.