Anyway, we recently had an issue with an Office 365 implementation that was preventing users who were accessing a shared mailbox from seeing all the emails. The problem was traced back to the "Focused" inbox. For the uses Inbox there was a clear option to disable this;
|Focused Inbox (outlook.com)|
|Shared mailbox (without Focused and Other options)|
On the plus side it's relatively easy to switch off the Focused inbox - but unfortunately, it has to be switched off for all mailboxes at once.
Click the settings cog at the top-right, and then the "Display Settings" option;
|Settings > Display settings|
Finally, click "OK".
And that's it, the "Focused" link will have vanished and you'll see all emails in your inbox.