I also have the same issue (Win 10 pro OS bld 19042.746). upgraded to Affinity Photo 1.9 (1.9.0.932) yesterday (after completing some focus stacks successfully). Now not working at all. With ~ 20 images it seemed to 'align merged sources' OK and then half way through the 'focus merge' the screen goes black with a few large blocks of color, which then disappear. The process runs to completion to give a black screen; in the Sources window the Focus merged image is completely black, the first image in the stack is speckled grey and the rest of the stack image thumbnails appear to show correctly.
Edit - just tried a small stack that fails above with the (GPU) Hardware Acceleration turned OFF (Edit->Preferences->Performance), it seems to work...
