I'm having the same problem with the focus merge function since updating to 1.9. I've tried several different stacks, and the result is the same: the bottom fourth of the screen is blank (black), often with colorful artifacts along the boundary between the blank bottom and useable portion of the final image.
Until this bug is fixed, is there any way I can revert back to 1.8.6? I can't find an option to download any previous versions, including1.8.6, anyplace on the Affinity website. It's all about the latest and greatest, whose glitches prevent me from doing what I need to do. This is a real inconvenience.