Moving a message from one folder to another with thunderbird, gmail-app on android and horde webmail has always the same behaviour.
The \Recent flag ist set and the \Seen-Flag is removed in the new folder. So it is not a problem of thunderbird.

BR Jakob

This is, what the logs say, when i move a message to another folder with thunderbird drag&drop

Dec 04 17:22:24 mymail dovecot[16982]: imap(user.name)<17010><XtHsqjR8JqbV0J0m>: copy from INBOX/Arbeiten/bla: box=INBOX, uid=849, msgid=<a133b9b2-6913-14e8-f509-b6c81750e076@bla.com>, from=Full Name <user@bla.com>, subject=TEST005, flags=($label3 NonJunk)

Dec 04 17:22:24 mymail dovecot[16982]: imap(user.name)<17010><XtHsqjR8JqbV0J0m>: expunge: box=INBOX/Arbeiten/bla, uid=7, msgid=<a133b9b2-6913-14e8-f509-b6c81750e076@bla.com>, from=Full Name <user@bla.com>, subject=TEST005, flags=(\Seen $label3 NonJunk)

So i think, there is a Problem with dovecot. Is it possible, that a misconfiguration causes such a behaviour?

It is only the \Seen flag which is removed and obviously the \Recent-flag, which is set, when i move a message to another folder. All other Flags can i set/remove with sieve-filters.
Maybe these flags are set correctly, but then is another process, which removes the \seen and sets the \recent flag on every new message in a folder...

BR Jakob

Am 04.12.18 um 17:02 schrieb Michael Slusarz:

On December 4, 2018 at 8:48 AM Jakobus Schürz <wertstoffe@nurfuerspam.de> wrote:

I have an additional information.

Moving a already seen messsage to another imap-folder (drag&drop in
thunderbird) shows this email in the other folder as unseen and recent.

Is this normal behaviour?

Depends on what Thunderbird is doing.  If it is doing a COPY, flags will be preserved per IMAP spec.  If it is doing an APPEND, then Thunderbird would be responsible for transferring the flags (and clearing the recent flag) to match COPY semantics.

michael