I have set my respawn using a bed but keep being sent back to spawn

I have spawned in a wither and have been dying a lot and I've noticed that I keep being sent back to my original spawn point (roughly 1000 blocks away from my base) from time to time with no reason. I have tried using the bed to set my spawn after dying and using a different bed but I'm still being sent away. The wither is not coming near my bed and it is not obstructed in any way that I can find.