While working on bug 37128, I notices field 335 is not in the default authority frameworks https://www.loc.gov/marc/authority/ad335.html I think if a field is not in the framework, and you import a record that has values in that field, the values will be deleted?
Oops, sorry, it was added in Update 31 which I did yesterday bug 37123 The fields are starting to be all mixed up in my head!