Unable to set a flag to an email - imap

Question

I have a mail that I cannot mark as seen or deleted from my inbox. (I don't know how I managed to have it…) If it helps, the server is a Zimbra instance.
I tried to set the flags by hand, to be sure to catch any error message. So openssl s_client …, LOGIN command, and then:
. SELECT INBOX
* 11 EXISTS
* 0 RECENT
* OK [UNSEEN 8] mailbox contains unseen messages
* OK [UIDVALIDITY 1] UIDs are valid for this mailbox
* OK [UIDNEXT 220360] next expected UID is 220360
* FLAGS (\Answered \Deleted \Draft \Flagged \Seen $Forwarded $MDNSent Forwarded $Junk $NotJunk Junk JunkRecorded NonJunk NotJunk)
* OK [PERMANENTFLAGS (\Answered \Deleted \Draft \Flagged \Seen $Forwarded $MDNSent Forwarded \*)] junk-related flags are not permanent
* OK [HIGHESTMODSEQ 425603] modseq tracked on this mailbox
. OK [READ-WRITE] SELECT completed
Very well. I'm in READ-WRITE mode. I check with another mail to be sure all is OK:
. FETCH 7 FLAGS
* 7 FETCH (FLAGS ())
. OK FETCH completed
. STORE 7 +FLAGS \Seen
* 7 FETCH (FLAGS (\Seen))
. OK STORE completed
Super! I marked this one as seen. But for the next one, with the same command, the \Seen flag is not set even if command succeeded:
. FETCH 8 FLAGS
* 8 FETCH (FLAGS ())
. OK FETCH completed
. STORE 8 +FLAGS \Seen
* 8 FETCH (FLAGS ())
. OK STORE completed
The same for the \Deleted flag. Is there a normal situation where, in READ-WRITE mode, I cannot put a flag on a mail? Or any test to check if something is wrong with my mailbox/mail?
EDIT, after arnt's suggestion:
I tried to copy this mail in another mailbox. The message is well copied, with another UID, is readable, and keeps the flags the former mail should have: for example, it has the \Deleted flag set when I select the mailbox (but it is not expunged, so I still can check that the copy succeeded).
. COPY 8 "TestMB"
. OK [COPYUID 411900 220324 220409] COPY completed
. SELECT "TextMB"
* 1 EXISTS
* 0 RECENT
* OK [UIDVALIDITY 411900] UIDs are valid for this mailbox
* OK [UIDNEXT 220410] next expected UID is 220410
* FLAGS (\Answered \Deleted \Draft \Flagged \Seen $Forwarded $MDNSent Forwarded $Junk $NotJunk Junk JunkRecorded NonJunk NotJunk)
* OK [PERMANENTFLAGS (\Answered \Deleted \Draft \Flagged \Seen $Forwarded $MDNSent Forwarded \*)] junk-related flags are not permanent
* OK [HIGHESTMODSEQ 425749] modseq tracked on this mailbox
. OK [READ-WRITE] SELECT completed
. FETCH 1:* (FLAGS UID BODY[HEADER.FIELDS (SUBJECT)])
* 1 FETCH (UID 220409 BODY[HEADER.FIELDS (SUBJECT)] {63}
Subject: Enligne_[...]
FLAGS (\Deleted \Seen))
. OK FETCH completed