Pistons can be powered by an air block below a redstone block. Placing a block next to it doesn't make it contract either. If the piston is placed before the redstone block, it won't extend until updated. It doesn't work in any other direction relative to the redstone block. The redstone block can also be removed and it will stay extended, but will retract once updated.

This actually works with any block powered by redstone

This bug is also present in 1.13.2