reset spawn point unable to update

when i got killed i didn't go to my spawn point that i set when i sleep, i went to the my first spawn point which is the fist time i load up the world. this make me unable to kill the wither boss cuz i was in a boss fight very far form it like 1000 block , when i die i have to took the nether protal to it, to continue the boss fight. the bug only happen like 3 time, i'm not sure after 3 time of walking back to the boss and the wither keep healing it self, i quit