Confirmed in SMP 15w41b. All portals broken both ways
Confirmed in Multiplayer. Oak wood stairs placed my players are invisible and they disappear for them after awhile. Also, other players will get stuck in the invisible stairs and can't get out unless they suicide.
Works in single player, not in multiplayer
Temporary fix: Break the portal in the nether, use a piston to push the shulker into the obsidian frame and relight the portal.