Piston head appears darker when mining

When breaking a piston/sticky piston extended or not extended the head appears darker

However, when it is extended, the head only appears darker when mining the body as opposed to the head.

Steps to reproduce:

1. Obtain a piston in survival mode (commands, etc.)

2. Place it down and start to mine it

3. Notice that the head is darker

What should happen: The piston head should stay the same color

What happened: The piston head appeared darker