I had the same issue [iMac Pro, OS 11.7.8, Affinity Photo 2, and Designer 2]
It was driving me nuts because the mouse worked in Basic and other brushes, but the pen didn't. It seems that Basic Brushes was the only Brush type that worked with the Wacom Pen, but other brushes did not work with the pen. Meanwhile the Mouse always worked with any brush type.
Try this: Go to system Preferences /Wacom Tablet, select Application Affinity Photo 2, under Pen tab Change "Tip Feel" to soft. - Same for Affinity Designer
Report back your findings.
