The diving behavior of the Ender Dragon when perching onto the portal was broken by the fixes of MC-87942 and MC-91858. Previously, the Ender Dragon would dive down to the portal while spitting acid. Due to the AI issues caused by the fix of the aforementioned two bugs, the Ender Dragon does not correctly dive downwards when flying towards the portal, causing it to make repeated attempts to fly to the center, flopping back and forth while slowly descending.

Relates to MC-271337

Note that this is not a duplicate of MC-197201, that report is refering to the divebomb attack that was removed upon 1.9's rework of the Ender Dragon. This report is instead about the Ender Dragon's perching behavior that was broken in 19w08b.