I have a large, sparse imap folder with read and unread articles.  In
recent versions of Emacs, when I visit this group, a set of articles in
the middle (when sorted by number) are marked as read with the
`O' mark.  I have bisected this to the following commit:

commit ea640581bad1c596f657ca405f6c97e1b4fc4b11 (refs/bisect/bad)
Author: Lars Ingebrigtsen <larsi@gnus.org>
Date:   Thu Jun 23 11:22:29 2022 +0200

    Ensure that nnimap doesn't send too long lines to the server
    
    * lisp/gnus/nnimap.el (nnimap-retrieve-headers): Don't send
    too-long lines to the server (bug#56138).

I've attached the FETCH command used by last "good" version and the ones
issued by a build with the offending commit.  In the "bad" version, the
articles from 60795 until 63651 are marked as read with an `O'.

Good: