The bug

While fighting the ender dragon, if two flowing blocks of water (from what I tested the bug occurred when the water was flowing down from the obsidian pillars) are in its path it will constantly fly between those two, the streams are close to each other for this bug to happen and the dragon will fly lower and lower until it clips into the ground. No projectiles will be shot from it nor will it try to go to the portal to perch. Occurs in both creative and survival, noticed from survival.

How to reproduce

Putting source blocks of water on two obsidian pillars close to one another, letting it flow down, and if the ender dragon gets caught in one it has a high chance of starting the bug. Examples are seen in attachments and below.