Base path for attachment summary view broken

Same situation: Drupal 8.6.17 - "Define the base path for links in this summary view" will not work (Paths on glossar-Items is )

Just ran into this problem with EVA. Argh.

I'm having a similar issue although it looks like the InvalidParameterException has been fixed.
Now the base path is simply ignored as if the value were ''.
My view has two glossary embed display, one for the given name and one for the family name.
I looked in
This is where the glossaries are rendered.
The first argument for the given name is passed into the first url parameter, resulting in a correct path of /a.
The second argument, which has a base path of 'all' is also passed into the first url parameter, resulting in a path of '/A' instead of '/all/A'.
This all happens in function template_preprocess_views_view_summary_unformatted():

// $view->args is already set to ['a', NULL] for the first glossary and ['A', NULL for the second glossary]
//For the latter, the base path has already been merged with the views arguments.
routeParameters: array[
  "arg_0" => "all"
  "arg_1" => null
  "view_id" => "my_view"
  "display_id" => "my_display"
]
But then parameter 1 overwrites arg_0 instead of being appended to it.
foreach ($route_variables as $variable_name) { // arg_0, arg_1
  $parameters[$variable_name] = array_shift($args);
}

The first parameter should not be in the first position, but the first free position.
Therefore, for this purpose at least, it can be fixed by appending parameters after the ones already set:

foreach ($route_variables as $variable_name) { // arg_0, arg_1
  if (is_null($parameters[$variable_name])) {
    $parameters[$variable_name] = array_shift($args);
  }
}

Finding the first argument already set with the base_path, it puts the first argument in the second position so that now arg_1 carries the first glossary letter A of the family name.

The temporary fix is to overwrite the theme function. Here's my gutted version

function my_module_preprocess_views_view_summary_unformatted($variables) {
  $view = $variables['view'];
  $argument = $view->argument[$view->build_info['summary_level']];
  $row_args = [];
  foreach ($variables['rows'] as $id => $row) {
    $row_args[$id] = $argument->summaryArgument($row);
  }
  $argument->processSummaryArguments($row_args);
  foreach ($variables['rows'] as $id => $row) {
    $args = $view->args;
    $args[$argument->position] = $row_args[$id];
    if (!empty($argument->options['summary_options']['base_path'])) {
      $base_path = $argument->options['summary_options']['base_path'];
      $tokens = $view->getDisplay()->getArgumentsTokens();
      $base_path = $argument->globalTokenReplace($base_path, $tokens);
      $url = Url::fromUserInput('/' . $base_path);
      $route = \Drupal::service('router.route_provider')->getRouteByName($url->getRouteName());
      $route_variables = $route->compile()->getVariables(); // arg_0, arg_1
      $parameters = $url->getRouteParameters();
      foreach ($route_variables as $variable_name) {
        if (is_null($parameters[$variable_name])) {
          $parameters[$variable_name] = array_shift($args);
        }
      }
      $variables['rows'][$id]->url = $url->setRouteParameters($parameters)->toString();
    }
  }
}

Additional question:
What if the second glossary had too many elements in the base path? This would break this mechanism. couldn't the base_path be populated automatically using the 'all' parameter for each of the preceding views arguments?

I believe that for displays not extending PathPluginBase, the summary option for arguments should not be available at least until this is fixed or a better solution is found.

I've added a basic solution as a draft, and I’d be interested to hear your thoughts on it.