If you have a color fill layer and try to export using "Quick Export As PNG" or "Export As...", a really strange bug happens. It seems like the mask on the color fill layer affects the dimensions of the exported image.
Take a look at the examples below. "Save for Web" works fine.
This is what happened when I tried to export my artwork.
I then removed the mask on the color fill layer. Now it works!
I then tried to see what happened if I changed the dimensions of the mask.