Blocks moved by pistons, also known as moving blocks, are immovable. Once the moving blocks get to their desired destination, they become a regular block again, and are movable. This is where the problem occurs. Sometimes the block can be moved again 2 gameticks (1 redstone tick) after the first piston moves the block, other times it takes 4 gameticks (2 redstone ticks) after the first piston moves the block, and if powered with a short redstone pulse, there is a random chance that the block does not get moved at all.

How to reproduce:
1. Place a normal piston (henceforth known as the first piston) and a block in front of the first piston. Then place another piston 2 blocks away with a 1 block offset (henceforth known as the second piston). The second piston will be facing into the area where the block is pushed to, not where the block already is. Build all of this within the same chunk. If you are confused how to build this, copy Example 1.png in attachments. Another setup that will produce the same result is Example 4.png.
2. Put a repeater on 1 redstone tick of delay facing into the second piston.
3. Put a block next to the first piston and the repeater, then place an observer behind this block.
4. Update the observer.

Observed Results:
The block will randomly either be pushed or not pushed by the second piston.

Expected Result:
The block will always be pushed by the second piston.