[KevinH]

After looking at the code for the epub3 metadata editor (just like the epub2 metadata editor), for safety it only allows you to edit the metadata tags it recognizes, ie. meta tags with known properties, dc metadata etc.

Yes you can create still custom metadata that will be stored in the opf using the metadata editor. But to then edit this unrecognized metadata, you would have to directly edit it in the opf, since you have added advanced tags that the Metadata Editor is unfamiliar with. But nothing should ever vanish from the OPF.

The main reason is that the OPF is pure xml, so custom attributes and custom tags would require adding the required namespaces and prefixes to the metadata tag itself. Sigil automatically handles that for opf, dc, dcterms namespaces (ie the ones it recognizes). If additional namespaces are required because of custom tags or custom attributes, the user would have to hand edit the OPF to add the appropriate namespaces and prefixes.