I agree with @jyeomans - I think the black you are seeing is the edge of the text box when cacheAsBitmap is not enabled. You can prove this by making the text box wider than the container and seeing if it is black.
This can be fixed by adding a blur of zero to the text fields (or your main container if everything is in a symbol).
Or you can update your template host files with the fix built-in.
Here they are pre-compiled with all my patches. These files should be dropped in your server folder and your template folder.
https://drive.google.com/drive/folders/1sdVZjC0UCBWbw5LpUQs4PXaRqojCjWGa?usp=sharing