Used a sticky piston to hold a chiseled stone block as a 1x2 door, used to teleport player once opened. found if a sign was behind the top chiseled block the sticky piston will fail to retract despite power input.

As shown in screenshots provided:

Screenshot 1: closed doorway, both top and bottom piston powered, and a sign is behind the top block in the doorway/

Screenshot 2: bugged opening, both pistons have no power yet the top piston wont pull back.

Screenshot 3: correct opening, both pistons have no power and no sign behind top sign.