Multisite setup ignore theme

Move back to NW due to MR still have no tests.

Up to this point I have succesfully used the following:

function hook_form_config_split_edit_form_alter(&$form, &$form_state, $form_id) {
  // Support both old and new versions of the module
  if (isset($form['blacklist_fieldset']['theme'])) {
    $form['blacklist_fieldset']['theme']['#access'] = TRUE;
  }else{
    $form['complete_fieldset']['theme']['#access'] = TRUE;
  }
}

This no longer seems to work after I followed the upgrade path to Drupal 10.2 and version 2.0 of config_split.
I am not sure whether this broke due to the updates that have been applied to the database, or because the code works differently now.
The patch of the merge request also still applies, so I don't know what is wrong, but all sites in the multisite system now no longer seem to understand that a different theme should be used.

I am investigating this, but maybe some one already knows the answer / has a solution?