This can happen in your main Outlook window, or when writing a new email, as Outlook decides which buttons to show on the ribbon. If there’s not enough room, then some are moved to an ‘overflow’ menu. You can see it in action by re-sizing the window. As you make the window smaller buttons are taken off the ribbon, and added to the overflow menu. Similarly as you make it larger buttons are added to the ribbon and removed from the menu.
You can show the overflow menu by clicking on the ‘…’ button on the right of the ribbon.
This shows the Mailock button and it works the same as if it was on the main ribbon.
If you want to show the button on the main ribbon just click on the ‘…’ button, right click on the Mailock option and choose ‘Pin to Ribbon’
The Mailock button appears, as you’re used to, on the left of the ribbon.
If you change your mind and want to put the Mailock button back on the ‘…’ menu, right click the button on the ribbon and click on ‘Pin to Ribbon’.