I have seen this happen when people have accidentally enabled Native Labeling and disabled the AIP add-in. The Information Protection bar is a feature of the AIP client.
This can happen if you have set UseOfficeForLabelling' Reg_Dword=1 rather than 'UseOfficeForLabelling' Reg_Dword=0
If that is the case you need to change the registry key under HKEY_Current_User\Software\Microsoft\Office\16.0\Common\Security\Labels
Then you should be able to use:
Set-LabelPolicy -Identity <Name> -AdvancedSettings @{HideBarByDefault="False"}
This may also be hidden if the user goes to Sensitivity and unchecks "Show Bar" manually, and in that case they will need to re-check it.