IMAP UID FETCH Command - Return two sequence numbers by one UID request - email

Question

In very rare cases server returns a second sequence number.
Example log:
C: A0020 UID FETCH 304244 (BODY[HEADER.FIELDS (SUBJECT FROM TO CC REPLYTO MESSAGEID DATE SIZE REFERENCES)] UID FLAGS INTERNALDATE RFC822.SIZE ENVELOPE RFC822.HEADER)
S: * 9 FETCH (BODY[HEADER.FIELDS (SUBJECT FROM TO CC REPLYTO MESSAGEID DATE SIZE REFERENCES)] {262}
S: Date: Thu, 29 Dec 2022 00:00:00 +0000\r\nFrom: Name <email#domain.com>\r\nSubject: Subject\r\nTo: <email#domain.com>\r\nCC:\r\n\r\n
S: UID 304244 FLAGS (\\Seen) INTERNALDATE \"29-Dec-2022 01:00:00 +0100\" RFC822.SIZE 226713 ENVELOPE (ENVELOPE DATA) RFC822.HEADER {7754}
S: MIME-Version: 1.0\r\nReceived: from ... email data
S: FLAGS (\\Seen))
S: * 11 FETCH (FLAGS (\\Seen \\Deleted))
S: A0020 OK FETCH completed.
Client request UID FETCH by UID "304244". The server responded that the email is "9" sequence number. At the end before "OK FETCH completed." the server returns another sequence number "11" with information about the flags.
I tried to find in https://www.rfc-editor.org/rfc/rfc3501#section-6.4.5 , https://www.rfc-editor.org/rfc/rfc3501#section-6.4.8 and elsewhere some information on such responses from the server, but found nothing. Does anyone know why the server in the "UID FETCH" command response returns another sequence number with information about the flags and what it can mean?
UPDATE:
Today I have got this case:
C: A0051 UID FETCH 305421 (BODY[HEADER.FIELDS (SUBJECT FROM TO CC REPLYTO MESSAGEID DATE SIZE REFERENCES)] UID FLAGS INTERNALDATE RFC822.SIZE ENVELOPE RFC822.HEADER)
S: * 40 FETCH (BODY[HEADER.FIELDS (SUBJECT FROM TO CC REPLYTO MESSAGEID DATE SIZE REFERENCES)] {209}
S: To: email#domain.com\r\nSubject: Subject\r\nDate: Tue, 03 Jan 2023 00:07:15 +0200\r\nFrom: <email#domain.com>\r\n\r\n
S: UID 305421 FLAGS (\\Seen) INTERNALDATE \"02-Jan-2023 23:07:20 +0100\" RFC822.SIZE 158940 ENVELOPE (ENVELOPE DATA) RFC822.HEADER {8278}
S: MIME-Version: 1.0\r\nReceived: from ... email data
S: FLAGS (\\Seen))
S: * 3 FETCH (FLAGS (\\Seen \\Deleted))
S: * 4 FETCH (FLAGS (\\Seen \\Deleted))
S: * 5 FETCH (FLAGS (\\Seen \\Deleted))
S: * 6 FETCH (FLAGS (\\Seen \\Deleted))
S: * 7 FETCH (FLAGS (\\Seen \\Deleted))
S: * 8 FETCH (FLAGS (\\Seen \\Deleted))
S: A0051 OK FETCH completed.
So, if I understood the answer below correctly. While the server run "UID FETCH" the email from the server, someone in another session change this and other emails. In begin of "UID FETCH" the sequence number was "40". But during command execution the sequence number changed several times: 40 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8. That is why the server sends me these "* number FETCH (FLAGS ...)" lines. Did I understand correctly?

Answer 1

What you want is described in IMAP-speak as: Your FETCH command directs the server to send an untagged FETCH command before the server concludes processing the FETCH command, and return a tagged response with the result (usually just OK).
Note the verb directs. Other things can also direct the server to send untagged responses, even at the same time as you direct it. For example, when new mail arrives, that directs the server to send an untagged EXISTS response as soon as possible.
Your example is a bit strange, but the server is allowed to do it.
The simplest way to deal with it is to maintain a cache of messages clientside, update the relevant message in the cache whenever you get a FETCH response, and when you receive a tagged OK, you can trust that the information you wanted is now present in the cache.
Make sure to test what happens if a message is deleted just as you request it. The server's untagged responses should cause your cache to contain something like 'this message has been deleted'.