Repro in Windows editor. Need a tablet (e.g. Wacom).

1. Open Input Debugger.
2. Pop open Mouse device.
3. Pop open Pen device.
4. Put the two side by side.
5. Move the pen around.

You'll see events streaming in for both pen and mouse. The pen device will update as expected. The mouse device will update its position only when the pen starts or stops moving.