This is because of the recent change in ff796823e5 with the hope that it
doesn't break other modes. But your bug report helped to reveal that it
causes breakage. So I had to revert it.
Gregory, could you please see if it can be improved to not fail in the
reported case? Additionally, on emacs-devel Zhiwei Chen said this:
It failed to work when buffer is auto selected via
`display-buffer-alist’. When the point moves to the *Occur* buffer,
typing “n”, “p” still sends “n”, “p” to isearch.
(defun display-buffer-select (buffer alist)
(let ((window (display-buffer-below-selected buffer alist)))
(when (window-live-p window)
(select-window window))))
(setq display-buffer-alist '(("\\*Occur\\*" (display-buffer-select))))
Maybe this could be handled as well?