Since before 1.9, redstone would connect to pistons when a transparent block with redstone was adjacent to the piece of redstone connecting to the piston, but not with opaque blocks.

This behavior has been used in several redstone contraptions for years, even though the visual connectivity was changed in 1.9 so that opaque blocks and transparent blocks looked the same, but with different behavior.

In snapshot 20w18, the previous behavior was broken, with the blocks behaving as if they were the same in this scenario. Such a change will break a multitude of older designs when live for the worse. I'd recommend that this gets fixed in a way that maintains the old behavior but with a visual that connects to the piston and the transparent block. I'm honestly kind of surprised I haven't been able to find anything in bug reports for this. A chunk of video describing this can be found here, but I've composed some screenshots below from versions 1.8.8, 1.15.2, and 1.16 pre8.