I saw the exact same bug report from two years ago. It says the bug has been fixed by 1.14.1, but now (1.16.5, 8/May/2021) the same issue is back again.

I'm currently mid fight with the Ender Dragon, and she's just flying around doing nothing and it's so hard to do damage to her like this. I thought it was a feature, but I never seen such thing before. And it sure is a bug (or I assume it is)

What happened before I notice the bug:
I believe I was at the stage of just slaying the dragon. The crystals were destroyed and stuff. Everything was working normally until my first death, I respawned right outside the portal and jumped in immediately. When I came in, the dragon was already going around in circle. How do I resolve this issue in the short run? I've ran out of arrows or any projectile tools.