Hi @Dan C
Thank you for picking this up. I am on a 2021 iPad Pro with Magic Keyboard running iPadOS 15.0.2, however, I managed to work out what caused my trackpad problem. (I am pretty new to the iPad. 😇) This only happened right after I enabled 'Touch for gestures only' in the Tools preferences panel, which actually disables access to many functions and features with the mouse/trackpad. They work fine when the option is turned off, but then it is very hard to use the pencil (2nd gen), as there is a pretty bad palm pickup present. I am not sure if it is even possible at all to differentiate between the two input methodologies - touch vs. mouse/trackpad - as I found that this works the same way in another vector graphic application on the iPad.
This might just be a hardware limitation. I guess I will just have to plan forward when I want to use the pencil and when I want to use a mouse and set my preferences in advance accordingly. Maybe turning touch (and trackpad/mouse) gestures only on and off could be made more accessible with a switch or button somewhere in the document window UI?