Outlook app Shared mailbox disappearing

Use ExcludeLastKnownGoodUrl to prevent Outlook from using the last known good AutoDiscover URL
HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Autodiscover
DWORD: ExcludeLastKnownGoodUrl
Value: 1

OR

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\x.0\Outlook\Autodiscover
DWORD: ExcludeLastKnownGoodUrl
Value: 1