You can put redstone on top off pistons without having a block between. But when you do this, in 1.11.2 it jumps off the piston when you deactivate the piston and in 17w06a it jumps off when you power the piston. I don't think that is intended. I have heard that piston disappear for a small second and appear than again. I think that is the problem why it jumps off.