Assigned configuration not marked as 'excluded' [#2605058]

Problem/Motivation

Scenario:

Admin is editing an existing feature.
New configuration is assigned by the assignment plugins.
Admin doesn't want that configuration to be packaged, so unchecks it and regenerates the feature.
Admin brings the feature up again for editing. Expected result: the removed items are not automatically assigned. Actual result: there they are again.

Relevant code section:

            // Save which dependencies are specifically excluded from
            // auto-detection.
            if (($section == 'detected') && ($default_value === FALSE)) {
              // If this was previously required, we don't need to set it as
              // excluded because it wasn't automatically assigned.
              if (isset($this->required[$component][$key])) {
                unset($this->required[$component][$key]);
              }
              else {
                $this->excluded[$component][$key] = $key;
              }
              // Remove excluded item from export.
              if ($component == 'dependencies') {
                unset($export['dependencies'][$key]);
              }
              else {
                unset($export['config'][$config_name]);
              }
            }

There is a parallel section for handling the 'required' constraint. The problem may be that these assignments are handled in the form generation code. So this bug may be solved by #2597850: Constraints (excluded and required) should be added when modifying a feature through Drush.