In order to import user defined fields into the library, you must create them first as the import function cannot create them on its own. If you miss some information you might have defined a wrong field type.
It is better to backup the library in MC11 and restore it in MC12. If you don't want to restore the whole library, make a clone library in MC11, delete its content, backup the empty library and restore it im MC12. In this way you will have the exact database structure as before.
And you can do a proper import of your music/image/video files without missing user fields.