Yep, that was it. Thanks

On Sat, Jun 16, 2018, 18:57 Eli Zaretskii <eliz@gnu.org> wrote:

> > From: João Távora <joaotavora@gmail.com>
> > Date: Sat, 16 Jun 2018 18:24:02 +0100
> >
> > $ ./emacs-master/src/emacs -Q
> >
> > Eval in the scratch buffer
> >
> >    (defun process-making-dummy-backend (report-fn &rest args)
> >      (make-process :name "dummy" :command '("echo"))
> >      (funcall report-fn nil))
> >
> >    (setq flymake-diagnostic-functions '(process-making-dummy-backend))
> >
> >    (flymake-mode)
> >
> > Now start typing something like this
> >
> >    (with-current-buffer
> >
> > In Emacs 26.1 the arglist of with-current-buffer is displayed in the
> > mode-line.
> >
> > In a recent master it is immediately cleared after the dummy flymake
> > handler kicks in, which is bug.  I think this started happening recently
> > but I dont have the time to bisect it.
>
> I think it's another manifestation of bug#31855.
>