During the wither fight, the wither is supposed to descend to the level of its target at half health or less (when it gains arrow immunity).  When I fought the wither the first time on 21w42a in my original world, I got it down to below half health and it was not coming down to my level.  However it still gained immunity to arrows making it impossible to kill with a bow or sword when it was below half health.  It regened enough health to regain its floating ability and it then I proceeded to reduce its health to below half for a second time and yet again, it still did not descend to my level making it impossible to hit with a sword while still being immune to arrows. (I attached a video where I retested it on a different world also on 21w42a and the same thing still happened).