Outlook.jp IMAP connection fails in Thunderbird and FairEmail

My Outlook.jp account works normally in Outlook Web, including sending and receiving emails. However, all third-party email clients, including Thunderbird and FairEmail, fail to establish an IMAP connection.

OAuth2 authentication succeeds, IMAP is enabled, and the server returns:

User is authenticated but not connected.

Please investigate whether there is an account-specific IMAP connectivity restriction or mailbox backend issue.