Same issue here, however, it seems like we're capturing the required reg key for our users to make it persistent, but can't confirm which one it is just yet.
The setting is off by default currently, hence our chase for the answer as well.
Our Default Apps and FTAs consists of the same, exception of what's highlighted green:
Toast notifications work if the user has opened Windows 10 Settings > Notifications & Actions either before OR after Outlook has been opened.
So it's clear something is triggering via the Registry, but can't be sure what yet...
Group Policy seems to be the better alternative to fixing this issue but haven't been able to confirm yet.