Here’s a video demonstrating the issue:
It wasn’t until I saw this video (which records the mouse pointer as a separate object) that I noticed that the limiting factor seems to be the sides of the PIE window. Changing it’s shape seems to change how much I can rotate the camera when RClicking, and if I release the RMB at one of those limits, the mouse is at the edge of the play window. Still unsure of what is causing this, or how to fix it.