I'm also seeing this issue in both Affinity Photo and Affinity Designer.
I have been given a screenshot with a gradient background, and I need to mock up a modified version of the image. As the previous poster suggested, in Photoshop I would use the Marquee tool select a 1px wide rectangle, then stretch it horizontally. If I try this in Designer, the left and right edges of stretched area fade to zero opacity. I have tried the methods suggested by @MEB of snapping to pixel layers.
Oddly, I was able to achieve the result I wanted once, but I'm not exactly sure why. And then, if I rasterize this layer, the gradient effect comes back - see the attached screenshots below.
If this isn't a bug, then could someone tell me how to achieve the effect I'm after?
Thanks,
- picklepirate