New outlook not saving as default email app on Windows 10

I've been using the new version of outlook but windows won't save it as the default mail app. I've altered it but on every restart it reverts back to an old version. This means that I can't send directly from other M365 apps such as word as it constantly uses the old version (which I don't have open) and therefore emails are just being held in those outboxes. 

Is there a way to fix this or is it a feature that's yet to be released?