Thunderbird forgets tags in IMAP mails immediately after adding them

Question

I hope this Stack Exchange site is the right one. Since a few weeks we are experiencing unexpected behavior with the tag/flag handling of Thunderbird. After assigning a tag to a mail, Thunderbird is marking it a very short time as tagged and after leaving the selection, the tag is gone.

I've enabled the Thunderbird log for IMAP

$ export NSPR_LOG_MODULES="IMAP:4"
$ export NSPR_LOG_FILE=~/tbird.log.txt
$ thunderbird

And grep'd the output:

tail -f tbird.log.txt | grep FLAGS

Sometimes it looks like:

[ ... ]:imap.strato.de:S-INBOX:SendData: 615 uid store 35045 +FLAGS ($label1)
[ ... ]:imap.strato.de:S-INBOX:CreateNewLineFromSocket: * 1026 FETCH (FLAGS (\Answered \Seen))
[ ... ]:imap.strato.de:S-INBOX:SendData: 619 UID fetch 35046:* (FLAGS)
[ ... ]:imap.strato.de:S-INBOX:CreateNewLineFromSocket: * 1026 FETCH (FLAGS (\Answered \Seen NonJunk $label1) UID 35045)

And sometimes it looks like:

[ ... ]:imap.strato.de:S-INBOX:SendData: 48 uid store 34991 +FLAGS ($label1)
[ ... ]:imap.strato.de:S-INBOX:CreateNewLineFromSocket: * 1022 FETCH (FLAGS (\Answered \Seen))
[ ... ]:imap.strato.de:S-INBOX:SendData: 52 UID fetch 35046:* (FLAGS)
[ ... ]:imap.strato.de:S-INBOX:CreateNewLineFromSocket: * 1026 FETCH (FLAGS (\Answered \Seen NonJunk) UID 35045)

without the flag "$label1" (its the original output).

If Thunderbird forgets the tag, after a restart the tag is set.

Can anyone tell me, if this is either a Thunderbird, provider or my settings fault is? I guess it's Thunderbird. How can i get more information?

I know another person who has the same problem, according to this maybe not a settings fault.

Btw. it's Thunderbird 24.4.0