Using New Outlook on Windows for professional communication and productivity
I have had the same issue, but I have the Office 365 Outlook app actually loaded on my computer.
It happened when I switched to the new view of Outlook. I am ready to switch back to the old view due to this feature not working. I use it all the time.
It shows the spelling error, but does not provide spelling suggestions in the right click pop up menu.
I checked the editor settings and these are all turned on. I also restarted the application.
In the new Outlook for Windows, simply clicking in a word which is flagged as misspelled will display spelling suggestions.