Sticky Pistons will not pull down slime blocks if immovable block preventing connecting block to move.

This is difficult to explain in a title. I have attached a screenshot.

Logically, both the Diamond Block and the Gold Block should lower with the sticky piston. The Slime Block should detach from the blocks to the right.

However, only the left example allow the Slime Block to be pulled down. This may have been reported but I could not find a bug that specified this.