Importing subtype-specific data to existing contacts loses their other subtypes

I just tried to import subtype-specific data for existing contacts, and their other subtypes went away. To recreate on demo:

1. Find a contact and set them to the 'Student' contact type
2. Note their contact ID
3. Find a custom field in a different subtype and pick a random value. Demo currently has 'Camera Skill Level' on the 'Volunteer', with possible values 1-5
4. Create a CSV with an ID column and a Camera Skill level column:

5. Import this CSV, setting Subtype to 'Volunteer', and 'Update'

6. Map the fields appropriately:

7. Check the original contact - they are now marked as 'Volunteer' and no longer Student.

This happens no matter whether the contact has the subtype originally - ie. if you are importing data with the purpose of inherently adding this subtype, or if it's already there.