I expanded the document to create a different aspect ration and a black band appeared in the image (where there was previously transparency). Note that when I exported the image to a PNG, the black band was not present in the image.
The original file was an EPS file.
Steps to recreate:
Open the EPS file
Open the Document Settings... dialog
Entered new vertical dimension to correct aspect ratio
Selected transparent background
Clicked on OK
Video of error creation
I am running Affinity Designer: 1.7.1on macOS 10.14.5