Yes, this was the issue, not the logic.

Changes have been made in Git (master):

commit fd9905ba596c00c5992b23026933b5cec69b285f
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Mon Nov 19 16:50:47 2012 -0700

     [mms] Fix preview message text in basic view (Bug #11724).

  imp/docs/CHANGES         |    1 +
  imp/lib/Mailbox/List.php |   25 ++++++++++++++++---------
  imp/package.xml          |    2 ++
  3 files changed, 19 insertions(+), 9 deletions(-)

http://git.horde.org/horde-git/-/commit/fd9905ba596c00c5992b23026933b5cec69b285f

Yes, if I separate the two "$v"-s, it works OK.

here $v comes from the foreach, later on it becomes a huge array:

foreach ($ids as $k => $v) {
     if (!isset($fetch_res[$v])) {
         continue;
     }

     $f = $fetch_res[$v];
     $v = array(
         'envelope' => $f->getEnvelope(),
         'flags' => $f->getFlags(),
         'headers' => $f->getHeaders('imp', 
Horde_Imap_Client_Data_Fetch::HEADER_PARSE),
         'idx' => $k,
         'mailbox' => $mbox,
         'size' => $f->getSize(),
         'uid' => $f->getUid()
     );

     if (($options['preview'] === 2) ||

Why do you think this?  $k contains the index of the message in the 
mailbox.  That number can change on any future page load (which is 
precisely why you are seeing incorrect messages being loaded).

$v is correctly used since it contains the UID of the message.

Now it shows previews from different messages :)

imp/lib/Mailbox/List.php:205

$v is used instead of $k as index
around 8 times

patch: changes the [indexes] to $k