I have the same problem.
My master.cf has no line mentioning "dovecot-lmtp".
My main.cf has line "mailbox_transport = lmtp:unix:/var/spool/postfix/private/dovecot-lmtp".
When upgrading Postfix, the process modified my "master.cf" from "lmtp unix - - n - - lmtp" to "lmtp unix - - n - - smtp".
I have the same problem.
My master.cf has no line mentioning "dovecot-lmtp".
My main.cf has line "mailbox_transport = lmtp:unix: /var/spool/ postfix/ private/ dovecot- lmtp".
When upgrading Postfix, the process modified my "master.cf" from "lmtp unix - - n - - lmtp" to "lmtp unix - - n - - smtp".