Sticky piston with an observer in front of it powered by a fast clock will act differently from 1.12.2

When a fast clock is powering a sticky piston with an observer in front of it facing away from it with one air block between the two, the piston sequence is different from one where the observer is replaced by a normal block or is facing in another direction.

https://www.youtube.com/watch?v=dBlhmo-8Pp4