cyrus-imad included with rhel8 is 3.0.7 and lacks tls 1.3 support
minor update version 3.0.12 was release in 2019 with added tls 1.3
https://www.cyrusimap.org/3.0/imap/download/release-notes/3.0/x/3.0.12.html
latest 3.0.x is 3.0.18 in 2023
https://www.cyrusimap.org/3.0/imap/download/release-notes/3.0/x/3.0.18.html

please rebase cyrus-imapd to at least 3.0.12 or better 3.0.x since it's a only a minor update in a declared stable train with minimal or no functional changes