When using a dispenser to spawn the wither, the achievement is not given. The achievement is given correctly when the wither is spawned by hand. The wither spawns correctly in both circumstances.
Comments 5
Singleplayer and multiplayer have, for a while, worked on the same exact game engine. Anything which applies on a server applies in singleplayer too, since singleplayer is basically just an internal copy of the server. Hitting 'Open to LAN' basically makes it so other people on the network can access that internal server.
It works as intended because, in multiplayer, there's no way to tell which player placed or activated the dispenser.
It is perfectly possible for the internal server to tell whether it is a singleplayer world. It already does this for example when pausing the game on singleplayer.
Saying that it should not work in singleplayer because of irrelevant and easily worked-around technical non-arguments is silly.
Closed as "works as intended"? While it may make sense not to give the achievement in multiplayer, surely it should be given in singleplayer?