bwuk vincent

First of all i want to say that i'm new to Affinity Photo so i could be the case that there is some misunderstanding on my part here. I noticed that when I insert PNG files, the only times where the Checkerboard background disappears, is when the resolution is 800x600. In all other cases the checkerboard background just stays and i can't find a way to remove it. Is there some setting that I have wrong or could someone explain to me how to remove the checkerboard in the other PNG files aswell?