Summary:

The repeater/comparator will not cause the required updates to blocks of a "manhatten" distance of 2, if there is no solid block in front of it. This can be seen with both droppers, dispensers and pistons, as they are powerable from a "manhatten" distance of 2 blocks downward.

Ways to recreate:

I will have provided a world download as well as a screendump from the world. In the world/screenshot, you should be able to see what setup can be used to visually see what is wrong. The pistons, which would update in most versions 16w42a and below will not in snapshot 16w43a.

Reason of case:

As the repeaters/comparators and observers updating was changed in snapshot 16w43a, this bug was introduced. The reason for it happening, is because of the fact that repeaters, comparators or observers will no longer update blocks with a "manhatten" distance of 2, if there is not a solid block in front of the repeater/comparator/observer. This is against previous intended behaviour, and is thought of as a bug.