Plugin ID 'json_api' was not found.

[OnGitHubSchatz]

Plugin ID 'json_api' was not found.

[OnGitHubSchatz]

I found a workaround by manually setting the plugin definitions with a hook cribbed from the 'plug' module:

function test_module_formatter_plugin_alter(&$plugins) {
    $plugin_manager = FormatterPluginManager::create();

    $lost_defs = array(
			'json_api' => array ( 'id' => 'json_api', 'label' => 'JSON API', 'description' => 'Output in using the JSON API format.', 'class' => 'Drupal\restful\Plugin\formatter\FormatterJsonApi', 'provider' => 'restful', ),
			'hal_json' => array ( 'id' => 'hal_json', 'label' => 'HAL+JSON', 'description' => 'Output in using the HAL conventions and JSON format.', 'curie' => array ( 'name' => 'hal', 'path' => 'doc/rels', 'template' => '/{rel}', ), 'class' => 'Drupal\restful\Plugin\formatter\FormatterHalJson', 'provider' => 'restful', ), 
			'single_json' => array ( 'id' => 'single_json', 'label' => 'Single JSON', 'description' => 'Output a single item using the JSON format.', 'class' => 'Drupal\restful\Plugin\formatter\FormatterSingleJson', 'provider' => 'restful', ),
			'json' => array ( 'id' => 'json', 'label' => 'Simple JSON', 'description' => 'Output in using the JSON format.', 'class' => 'Drupal\restful\Plugin\formatter\FormatterJson', 'provider' => 'restful', ) 
    );

    foreach ($lost_defs as $base_def) {
        $plugin_manager->processDefinition($base_def);
        $plugins[$base_def['id']] = $base_def;
    }
}

Still no clue why doGetDefinition() fails to find anything for plugin formatters specifically in my environment.