Discovered this behavior while experimenting with something relevant to a mailing list discussion.

1 - Create any convenient image and get your tablet ready.

2 - Using the mouse (Core Pointer) and a painting tool, draw a few strokes on canvas.

3 - Then do some painting/drawing with your tablet pen.

4 - Now, with the pen cursor still located inside the image area, lift the pen away from the tablet and set it aside.

5 - Switch back to your mouse and start painting again.

At this point, GIMP's brush outline doesn't track the cursor's movements and nothing is performed when you try to paint.  However, if you mouse out of the image area and back in, the mouse starts working again!

Step 4 is very important - if you pen outside the image window before switching back to the mouse, everything works fine.  Apparently when the pen is no longer active, GIMP waits for a "mouse enter" event on the image window before it starts listening to the core pointer again.  If the cursor was left inside the image window when switching from pen to mouse, said event doesn't fire so GIMP acts as if the pen is still in use and ignores the mouse.