Importing contacts clears out preferred communication method

Updating any contact through a CSV import sets preferred communication method for those contacts to null, even though preferred communication method was not included in the import.

How to replicate:
0. Go to a contact (e.g. ID 1) and set preferred communication method to Email and Mail
1. Create csv file with three columns: contact ID, first name, last name. Populate with 1 row (1, john, smith)
2. Import CSV file using "update" duplicate matching rule, and matching the three above fields.
3. After import, the first/last name is updated, and the preferred communication method is null.