Hi,
I am an app developer and I use Affinity Designer to do all my App Store screenshots. The way I do it is I use a document with many artboards, each representing a screenshot and a device (and possibly a language version, but that might be pushing it). Finally I use the export persona to export the artboards as images to be uploaded to the App Store.
The problem I'm having is that if the artboard location for a single screenshot is not a "full pixel" but instead has decimals the image/slice size in export persona might be one pixel less or more than the actual artboard size. This is problematic since App Store requires exact resolutions. Figuring out the fix/workaround took me a long time but it still is a chore to ensure every artboard to be on a full, non-decimal pixel location before exporting.