If a user customizes flycheck-display-errors-function to set it to
flycheck-pos-tip-error-messages, or if they simply "save" the
flycheck group customizations while flycheck-pos-tip-mode is
enabled, flycheck-pos-tip will malfunction on the next Emacs
session.

In particular, flycheck-pos-tip-mode will not add the correct hooks
to dismiss the error tooltip because
flycheck-display-errors-function is already set to the expected
value.

No matter whether flycheck-pos-tip-mode is invoked on the next Emacs
session, the errors are displayed by tooltip but the tooltip does not
get dismissed automatically as expected.