Summary
Observers no longer cancel their tile tick when moved.

Expected behavior

When an observer is powered, or about to be powered, and it gets moved by a piston the tile tick should cancel, as it does in versions before. 

The bug

The observer does not cancel the tile tick when moved, but keeps it.

Why this should be fixed

The observer canceling its tile tick is extremely useful for redstone. It is widely used to make fast observer clocks, and essentially all observer based flying machines rely on some form of this behavior. The behavior is not harmful in any way, the new behavior on the other hand breaks a lot of existing contraptions. 4 gametick clocks are even more widely used than the behavior in flying machines.