A redstone signal can pass through a piston like how it would be if you powered a block by repeater / redstone torch, but only when the piston is retracting.

This is similar to the bug reported here, except the examples shown there no longer work in 19w02a.
Except I found cases where the bug still works, as shown in the video below.

What I expected to happen was...:
Redstone signal should not have passed through the piston

What actually happened was...:
I could retrieve a redstone signal output from the piston, but only during the piston's retraction.

Steps to Reproduce:
Video shows examples and easy redstone contraptions to reproduce bug.

Youtube Video

More examples can be found in ilmango's video: https://www.youtube.com/watch?v=XxMa0O5rAiw