The getInstance method of Joomla\CMS\ToolbarToolbar is depreciated in 5.0 in favour of using the Container to obtain the toolbar object however mod_toolbar.php still uses getInstance() which is incompatible if the Toolbar as already been obtained from the container.

Steps to reproduce the issue

In any admin view that adds a toolbar to the display, replace the line

$toolbar = Toolbar::getInstance('toolbar');

with

$toolbar = Factory::getContainer()->get(ToolbarFactoryInterface::class)->createToolbar('toolbar');

Expected result

Refresh the admin view and the Toolbar should appear as it normally would.

Actual result

An empty toolbar is rendered, nothing is displayed.

System information (as much as possible)

The getInstance method of Joomla\CMS\ToolbarToolbar checks for the existence of an $instance of a toolbar object and if not found it will will create a new $instance object from the container. However if you have already created the toolbar object from the container and primed it with your buttons, say in a HtmlView, then the getInstance() doesn't recognise that you already have a toolbar, because it is not in $instance and thus it creates a new empty toolbar object from the container.

The new empty toolbar is then what is rendered by mod_toolbar.php and similar processing is found in a couple of core template files.

Any situation where a subsequent reference to getInstance() occurs to check for an $instance of a toolbar object would also create an new object if the original toolbar was created from the container.

public static function getInstance($name = 'toolbar')
{
    if (empty(self::$instances[$name])) {
        self::$instances[$name] = Factory::getContainer()->get(ToolbarFactoryInterface::class)->createToolbar($name);
    }

    return self::$instances[$name];
}

Additional comments

Once I found the cause of my issue and knew what I was looking for I was then able to find other references to problem therefore it is known but I couldn't find any specific issue raised to address fixing the problem. By raising this issue other people who are paying attention to their code and the depreciated methods may find this and quickly understand why their toolbar(s) aren't displaying.

This problem gets mentioned here, https://issues.joomla.org/tracker/joomla-cms/39537#event-717209

and here at the Toolbar::getInstance() three quarters the way down the page, https://docs.joomla.org/J3.x:Developing_an_MVC_Component/Upgrading_to_Joomla4

A temporary solution is to render the toolbar yourself in the the /tmpl file with
echo $this->toolbar->render(); and if placed inside an element with an id="subhead" it should be displayed in the same style as the mod_toolbar.php rendering.