Attached is a screen shot of AP 1.7, with a picture loaded. It displays with horizontal stripes:
I am not aware of having created these, nor do they show when I export the file to jpeg:
I have not been able to reproduce the issue, but I have kept the history saved with the file. One unusual thing - for me - is that I applied tone mapping. The problem appeared after I cropped.
Affinity Photo 1.7.0, iMac (Retina 5K, 27-inch, 2019), Radeon Pro 580X 8 GB, MacOS 10.14.5 Mojave