I recently had an issue with Outlook 2021 and Outlook 365: Some of the favorites (located in the left pane on the top) were disappearing on Outlook restart.

I found some suggested solutions to start outlook from the commend line: outlook.exe /resetnavpane. But that did not help in my case.

What worked for me was to add the following Registry key and then restart Outlook:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Autodiscover
DWORD: ExcludeLastKnownGoodUrl
Value: 1