We're using ArcGIS Maps SDK for Qt v200.6.0 and we're rendering symbols using a dictionary renderer and a .stylx file.
For a specific symbol we'd like to add a background to a text. In ArcGIS pro we edited the stylx file's symbol to look like this:

For this we defined a "background" callout on the Text Symbol like this:

However, when we then render the symbol using ArcGIS Maps SDK for Qt v200.6.0 only the text is rendered without the red background.
Is a background callout not supported in ArcGIS Maps SDK for Qt? Is there a workaround that works with varying text lengths?