- Either its not shown on all client
- or its shown one one client but on another not
First you need to have activated Outlook with an account related to a dedicated Office 365 tenant where focused mailbox is enabled on a Organization Level.
Even if one client shows up focused mailbox and another not, simply changing the accounts to the same subscription account will not activate the focused mailbox.
It all comes back to the same solution:
You must activate it on your Exchange Org:
Get-OrganizationConfig -FocusedInboxOn $True
Login to Exchange Tenant, checked Org Level (undefined) and user was enabled.
Restart Outlook and it behave as expected