Hi Chris, same problem here, I just bought Affinity Designer a few days ago, and started testing it with my tablet today,
I did try this actually, but it didnt work.
The other thing is my tablet is kinda old, so the wacom preferences software doesnt even show the Windows Ink button (Although I did check with Wacom and downloaded the only driver for my tablet that works with W10), I have a CTH-670 (Bamboo Create).
Driver: 5.3.5-3
I haven't tried the beta version, so I'll do that right now.
EDIT:
Alright, so with the latest beta 1.8.4.650, the problem seems to be fixed, even with Windows Ink turned on over in the settings section. 👍