Data elements from "personal" object deleted upon update

I created a record using /user-import, then updated it using /user-import. When I ran the update json through /user-import, I included only the required data elements (username, externalSystemId, and lastName because the personal object was present) and the actual update, which was the addition of a middleName in the personal object. I also set the "updateOnlyPresentFields" element to "true".

After I submitted the update to /user-import I did a GET on that user's record to compare the before and after. The middleName had been added as supplied in the update json, but all of the elements of the "personal" object that were not included in the update json, i.e.firstName, email, preferredContactTypeId, and phone, were no longer present in the record. I can only assume that they were deleted by the update process because they weren't present in the update json, even though updateOnlyPresentFields was set true.

I conclude, then, that when using /user-import for update, the "personal" object must be completely reconstructed or its elements will be deleted. Is this expected behavior?

I have attached an MS Word file that shows the json at each stage of the process.