Strider hero does not work with mulitplayer

I was playing the new map with friends and we got all the way to the wither skeleton boss, we killed him and nothing happened even though there should of been another boss. I went into spectator mode to find the second one on the roof and not spawning where he should be and even after killing him did not have an effect as we were still in the same place and did not go to another cutscene or open the new area,

The last picture has a hole in the wall cause my friend thought there was something Behind it so we could keep moving on.