Yes. When you update users via bulk import and leave fields blank, any values associated with that blank field are removed from the user's profile.
The only fields that do not change when left blank are the newPassword
and newExternalUserId
fields.
We recommend that, when updating existing users, you select Download list instead of Download template. When you download the list, it includes all current information for all of your user accounts, and is easier to make deliberate changes to only the required areas.