Not recieving the 'The Beginning' achievement for killing the Wither boss

I have killed the Wither twice now in a multiplayer world I play in with my friends but do not own. I recieved the achievement for spawning the wither but not killing it. I made sure I was close to the Wither when it dropped the nether star. I have had no other issues with any other achievements unlocking. I tried exiting and joining back in the world and signing in and out of my microsoft account but it still didn't change anything. I fought the wither by myself the first time and with another player the second time both on the same world.