An addon domain or subdomain's SSL certificate is not returned by the Dovecot service when making an IMAP or POP3 connection.
Dovecot uses the /etc/dovecot/sni.conf for the SSL configuration. The domain will not be added to the configuration if there are no email accounts.
Create an email address to the addon domain or subdomain:
How can I create an email account?
After, rebuild the Dovecot SNI configuration: