The ender dragon will come to the center of the end and hover over the bedrock portal frame and breathe acid as usual. When he's done. sometimes he fails to fly away. He stays there, flapping his wings slowly as if he were flying. Approaching him in survival causes knockback and damage, as if he flew into you. He is also vulnerable to bow shots, to which he is supposed to be invulnerable while on the frame.

--What I expected to happen:
The dragon comes to the bedrock frame, breathes acid for a while, remains invulnerable to arrow shots, then gets up and leaves. The battle proceeds as normal.

--What actually happened was:
The dragon hovered over the frame as normal, breathed acid, but instead of taking off and flying around, he continued floating above the end portal. His wings started flapping more slowly, as if he were flying, and he became vulnerable to arrows, as if he were flying.
Footage of the bug in action: https://youtu.be/yEj8Z0Y-2qI?t=1050

--Steps to reproduce:
Since this seems to happen at random, the most I can say is:
1. Go to the end.
2. Fight the ender dragon for a while. Pay close attention to him when he is over the bedrock breathing acid.