I am also having difficulty with focus merge in Affinity Photo   1.5.1.54.   It is loaded on my Dell Precision M3800 with Windows 10 Pro operating system.   The fault occurs whether I am using RAW files or jpeg; and whether I am using mouse or graphic tablet to manipulate.   Focus merge appears to work successfully until you wish to enter the touch up mode.  The merge process works as expected, and the merged file appears on screen and at the head of the source list. But if I try to do any