I'm not sure why the IR sensor was so obscurely placed. Maybe to reduce sunlight interference, or to keep it close to the circuitboard, but I found that it made the robot less responsive, and I suspect that is why I've seen many videos of Bittle with the tail removed.
So, I made a new tail from EVA foam (because I did like the shape), and extended the sensor to the tip of the new tail.
Demo video: https://youtu.be/4_mBuNrDS4Q
Good DIY! Never imagined😀