The problem is with the Crop Tool, if you select the option absolute dimensions and chose any resolution the app can’t scale the select resolution, for example if you have any file with 4000 x 3000 px with 300 dpi and you chose 800 x 600 px the app should show small square into the picture with 800 x 600 px, but the square leaves the area of picture by a large number of pixels.
The Random close happens in any moment you work with RAW files in RAW Persona I use a Nikon D7500 and my device is a iPad Air 3Gen and app is a Affinity Photo 1.7 for iPad
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