The summary explains the issue. What I mean of weak redstone current is, when you give direct redstone current to a block with redstone. Repeaters should catch it and it works with other blocks but not with pistons.

What I expected to happen was:

Repeater should catch the weak redstone current from a block like in the screenshot.

What actually happened is:

This does not work with pistons.

I don't know if this is intended...