What I expected to happen was...:
The piston should move the 2 concrete powder up and the redstone signal should pass through until the piston retracts

What actually happened was...:
The piston extended and the concrete powder moved upwards, but didn't fall back on the piston but floated and the bottom powder disappeared, but you can't walk through (it's like a ghost-block). The redstone signal can pass through now, but permanently.

Steps to Reproduce:
1. Place a piston facing upwards
2. Place two blocks of concrete powder on top of the piston
3. Place two blocks opposite each other (with the powder in the middle of it) next to the top concrete powder block

4. Place a repeater on one of that blocks facing to the concrete powder and power the repeater.

5. Place a redstone dust on the other block.

6. Now power the piston

Show on YouTube (because the file is too big): https://youtu.be/zyKyOkrgxKc