[2.0.0-beta1] Field formatter process text does not appear processed

I will have a look

Hello, thank you for reporting this bug.
Pierre, i saw you took the issue. i think i may know where it comes from.
When we have field_property which prop_type is "string" (like the processed text we are talking about), the getPropValue from the plugin is returning the data.

when the plugin value is used in slots, it goes into the convert mechanism, and the value is then encapsulated into a
[#plain_text => ]

in Drupal\ui_patterns\Plugin\UiPatterns\PropType\SlotPropType :

public static function convertFrom(string $prop_type, mixed $value): mixed {
    return match ($prop_type) {
      'string' => [
        '#plain_text' => $value,
      ],
    };
  }