Seems we need to clear some caches on form save

@grevil any suggestion which it should be? Container maybe?

Could also be \Drupal::cache('config')->invalidateAll();.

It would even be better, if we could directly invalidate the user_registration_reminder config, although I am unsure how to do this, as "invalidate()" requires a cache id. Might be the same as the settings id, or this is used for an entirely different "config". Not sure, we should test this.

Thanks @grevil I think other modules should have the same issue in their config forms? I'm even wondering why core doesn't handle this through https://git.drupalcode.org/project/user_registration_reminder/-/blob/1.x...?

Or do we have a typo or mistake somewehere?

Maybe this helps? https://www.drupal.org/node/2406455 →

But thb. I don't even understand why this is cached at all, I mean this is not about a render array or something... and we're changing config through Drupal's API - so the API should be able to handle this.
Maybe we should first ensure this is really a caching issue...

This could be interesting!

      $email_update_status_emails = $form_state->getValue('enable_update_status_emails');
      if ($email_update_status_emails) {
        // Reset the configuration factory so it is updated with the new module.
        $this->resetConfigFactory();
        $this->config('update.settings')->set('notification.emails', [$account_values['mail']])->save(TRUE);
      }

(From SiteConfigureForm(), "submitForm()" in core)

Hm... or maybe not, not idea...

The implementation suggests, that this won't help:

  /**
   * Resets the configuration factory.
   */
  public function resetConfigFactory() {
    $this->configFactory = NULL;
  }

Mhm I really don't think that should be needed...

Okay for now let's postpone this until we have an idea what needs to be done in detail and why. The effect is minimal, I think.

This is what AdminToolbar does in their settings form:

    parent::submitForm($form, $form_state);
    $this->cacheMenu->invalidateAll();
    $this->menuLinkManager->rebuild();

So honestly, I would start with \Drupal::cache('config')->invalidateAll(); and then give some other potential cache bins a shot:
https://gitlab.com/webksde-public/drupal/drupal-snippets/-/snippets/3636....