Affinity Photo & Designer 1.8.3 (App Store installed)
macOS Mojave 10.14.6
On my MBP either program was unable to open any editable file from anywhere on the file system nor could it save a new file that was created, resulting in a warning of "Permission Denied."
However, on my Mac Pro, everything worked just fine. After some research someone said it was related to the number of fonts installed on the system. I confirmed this case by reducing the number of fonts to match my Mac Pro on my MBP.
MBP had 2,800 fonts and the MP had 2,300 enabled.
Also, an interesting side note, Publisher which I've purchase direct instead of the App Store had no problems with 2,800 fonts.
No other program is affected by the number of fonts enabled, this bug should be fixed.