I would the mask to completely hide the image where full black is use -on the outline of the image.
Instead of hiding the mask blacked part, it always display the image in a translucient fashion.
Here a rendered PNG:
How to make my mask fully hide the blacked part?
You can appreciate in the joined file -the joined file is noted .afpub, I have created the original composition in afpub and lastly tested the file in afphoto also.
After recreating the case in Affinity photo seems it works now. Maybe a bugg of some sort with the original composition created in Publisher or other, I let you appreciate.
translucient_mask_.afpub