->collapsedSidebarWidth() not taking effect

<?php
class AdminPanelProvider extends PanelProvider {
  public function panel(Panel $panel): Panel{
    return $panel
      // ...
      ->sidebarWidth('16rem') // Has effect on the full sidebar
      ->sidebarCollapsibleOnDesktop()
      ->collapsedSidebarWidth('9rem'); // Has no effect at all
    }
}