I am playing on bedrock survival (1.16.10) and spawned the Wither underground. It killed me first time. I am able to get back my items, back off, and quit the game. I came back more prepared second time and able to beat him until zero health. I continued damaging it with my sword but it just won't die and eventually killed me again. I quit the game, restored backup, try killing it again but same thing happened. 

https://drive.google.com/file/d/18t08kjD_JfjO8cNcXcP263Q-plWdrV54/view?usp=sharing

Here is the video I manage to capture today. Similar to prior, but this time the Wither's health bar became empty when it reached half health, which is slightly different compared to prior.

I've tested in another new world and able to kill it as expected. Seems like something wrong with my first world but I don't know what's wrong. It is a plain survival vanilla world. I didn't turn cheat on, install any mod nor done anything weird to it. This is bad with an unkillable wither inside because I've spent a few years playing this world in survival.