Shady piston shaft texture

Not that serious, but I assume players playing on snapshots are expected to submit every bug they find, so... I was just randomly playing around with redstone and pistons, when I noticed that extended piston shafts that have a block placed at their pushing direction(like on pictures) get that... shadowy? texture. I don't know if it is the case on all the versions but it happens also on 1.15.2, or if maybe it's not a bug but a feature(in which case i'm really sorry for posting it here). Also apologies if it has been posted before, but I couldn't find that in the list of existing posted bugs.