Hi Lars,
you can try to use the method "updateWithSubDeletion" or "updateDocWithSubDeletion": If you explicitly leave a field away, the value is getting nulled (not set to the DI API default).
Make sure, you retrieve the object via ExportImportMode first and copy all relevant fields and segments to your update payload (just take away the field you want to set towards NULL value).
Best regards
Bastian