For certain user, while on UI15, they are not getting ServiceNow Header in their profile. Neither Logout/Global Search option is visible.

The issue that you are seeing is actually due to a personalization made on the user level. In UI 15, there is an option to collapse the banner frame and this is done through the tiny arrow right beneath the banner frame itself. The same can be used to expand and show the banner as well

Please follow the image below for reference and click on the arrow shown on the image while logged in as/ impersonating the affected user.

The following steps can be taken either as Admin (by impersonation) or by the user themselves after logging into the instance:

1. Login (or impersonate, if you are an admin) as the affected user on the instance
   2. The Banner frame on the top does not not show up for the user, however, notice that there is a tiny arrow on the middle (please refer to the image below)
   3. Click on the arrow to have the banner frame enabled