"The Beginning" Achievement Bug

So I was battling the wither at the top of the nether since my friend said it was the best way to get rid of the wither asap. As I defeated the wither ofcourse, i got the nether star and I was wondering why I havent recieved my achievement yet since achievements are my goals on minecraft. I did defeat the wither and used the nether star to craft the beacon yet the achievement still havent showed up. I dont think the realms is in creative mod nor the cheats was activated so i hope this issue is ought to be fixed..