How to reproduce:

1. Place down a fence post and place an observer facing that fence post

2. On the opposite side of the fense post, place an observer with its detector side facing upwards (the fence should connect to it)

3. Move the upwards facing observer down with a sticky piston by a 1 tick pulse

4. The observer looking at the fence post will not detect it changing shape

In any other observer orientation or pulse length, the fence change gets detected