A profile for an Office 365 shared mailbox is unable to see any Office 365 groups by using Microsoft Outlook or Outlook on the web. This occurs even though the shared mailbox is a member of the Office 365 group.
This is by design as Office 365 groups don't support explicit logon. Shared mailboxes don't have an enabled user account that's used for logging on.
Convert the shared mailbox to a regular mailbox. For information about how to do this, see Convert a Mailbox.