If you look at the gpg-enable-keyboxd.bat script in gpg4win-4.2.0 you will notice that error handling for the import is commented out because for me it would always return an error code.
gpg --export | GNUPGHOME=$(mktemp -d) gpg --import || echo fail
Adding things like
--export-options backup --import-options keep-ownertrust,bulk-import,restore
does not have an effect.
It always exited with an error code but no apparent error.
I can provide the keyring in question to anyone who wants to look into this.