So in the end in my world I fought withers in the version 1.16 and when I couldn't kill it I used to put it on peaceful because I didn't want to lose my progress. I also had problems with withers where if it reaches stage 2(when you have to kill it with a sword) and the fell in the void.   After I updated my game to 1.17 a wither keeps respawning (probably the one I despawned or the one that fell in the void) at an area a little away from my main end island. It spawns again after I kill it or if put it in peaceful (and back in hard mode) and join the game again. 

Wesley Lourens