Hello,
I ran into this bug while trying to retouch this photo I snapped in the Bone Church of Kutna Hora (the one decorated with human remains).
My plan is to make that "hands off" sign on the right disappear by copying a section from the left, flipping it horizontally and pasting it. But when I attempt to move the selection to it's destination on the right, it disappears behind some viewport and stops me in my tracks. I've captured a video of the behaviour:
So here are the steps I'm taking:
Create a selection.
CMD+C then CMD+V to copy the selection and paste into another layer.
Press V to activate the Move Tool.
Right click the selection and choosing Invert Horizontally.
Attempt to move the selection to its destination, where it vanishes.
I can reproduce this behaviour with any photo I open.
Affinity Photo version 1.7.3, MacOS Catalina 10.15.1, MacBook Pro (Retina, 13-inch, Early 2015) with 16GB RAM.