eftee74

I have the same issue. See three examples with luminosity range mask in preview mode.
Mask in the first picture is ok, but as soon as the curve touches the uppest x-axis (second picture) pure white becomes suddenly black .
I'm able to partialy workaround it moving the most right node to the left. The curve in the third picture is very close to the second one, but pure white remains as is.

Ahead of the curve? This is literally the only significant piece of software I use that chose to deliberately implement Microsoft's horrible tablet centric interface-in-an-interface and totally break the desktop experience.
It was an appallingly bad decision for numerous reasons, some of which have been highlighted in this thread.
I recommend the following fix that you implement ASAP in a maintenance release: when installing, create a stub executable in C:\Program Files\Affinity\Photo that contains a copy of the icon and invokes the executable stored in C:\Program Files\WindowsApps\<mangled installation folder name>\App\Photo.exe. Rinse-repeat for Designer and Publisher. In this way, the applications will be accessible using the standard desktop interface metaphor of Windows as well, instead of only via their ridiculous Windows App model that nobody I know even uses. 
 

Capture One is not impacted by not being able to access the .exe file as it relies on an application registering itself as an image editor which automatically registers it in C1's "Open With" plugin.