The (doveadm search HEADER "Header" "") command does not return email which have the header "Header"

Hello there,

When using the doveadm command (search HEADER "X-Discourse-Post-Id" "") or the IMAP command (SEARCH HEADER X-Discourse-Post-Id ""), emails containing a X-Discourse-Post-Id header are not listed.

I've also tried with non X- headers, such as List-Unsubscribe and Auto-Submitted.

RFC 3501 1 and Dovecot's documentation 2 both says that this command should return emails containing the header searched.

I couldn't find a mention of this bug in the git history, hence why I'm posting here, maybe there is something wrong with my installation ?

I'm running Dovecot 2.3.20 on Debian on amd64 with an ext4 filesystem. I attached the output of dovecot -n.

-- Cordially, Léo