Block plugin's render array values ending up in $variables['content'] instead of $variables
This is how block templates work, they render the variable content.
{{ content }}

Your code though is using a block template inside of a block, which doesn't make sense, because the block template is wrapped around the block by Drupal automatically. You probably want to build the inside of the block like a controller output by using a custom template not derived from a block template, which you do when you don't start the template name with block__.
See https://www.drupal.org/docs/theming-drupal/twig-in-drupal/create-custom-twig-templates-for-custom-module