When there is a multiple userdb to look for user, Dovecot log «auth: unknown user» when the first userdb search failed. Then Dovecot successfully search user in the second userdb and things works. But "auth: unknown user" event left in a logs.
Logs look like this:
Apr 1 12:38:48 mail dovecot[75384]: lmtp(75397): Connect from 127.0.5.1
Apr 1 12:38:48 mail dovecot[75384]: auth: ldap(order@XXX,127.0.5.1): unknown user
Apr 1 12:38:48 mail dovecot[75384]: lmtp(order)<75397><D8m2HCjIRmKFJgEAABdyCg>: msgid=<157609214.20220331172309@YYY>: saved mail to INBOX
Debug show failed search in the first userdb and successfull search in the second userdb.
When user found in the first userdb «unknown user» event does not appear.