display_name build incorrectly on profile changes

1. Edit your contact so that is has first, middle and last names. display_name contains all three.
2. Go to Drupal's 'My account' -> Edit -> Name and Address and Submit. display_name now is built just from first + last names.

Expected result: display_name should be built consistently.
Actual result: Middle name is not present after saving one's data in Drupal's 'My account' (although the middle name is still in the db).

(Ideally, display_name would be just a virtual column build dynamically through a VIEW, but I assume we use display_name for efficiency reasons, so that's probably not really a valid solution.)