One instance of broken AI is with how the Ender Dragon descends to the fountain. The Ender Dragon is supposed to dive down to the fountain, but in versions after 19w08b, the Ender Dragon instead flops back and forth slowly descending in an obviously buggy manner. The Ender Dragon also seems to very rarely descend to the fountain unless all of the crystals are broken, something that it did more often in versions before 19w08b.
Another instance of this broken AI is with how the Ender Dragon phases are broken. When the dragon is supposed to be circling the arena it instead will do that same buggy flopping back and forth motion it does when descending to the fountain, or it will simply fly in a circle/triangle over and over. It will do this between two places over and over until it decides to either fire a dragon fireball, charge a player, or it decides to descend to the fountain. This part of the broken AI can be seen best when comparing the after 19w08b AI to the before 19w08b AI directly side by side, especially since in versions before 19w08b the Ender Dragon flies up and down smoothly and fluently when circling the arena, while in after 19w08b the Ender Dragon tends to just stay on the same y-value when circling the arena. It also just seems to fly faster and more fluently in general in versions before 19w08b.
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Please make sure you can reproduce this without any modifications installed, using Optifine invalidates bug reports, no matter the context.

You can't be serious man. Optifine is a purely visual modification. At this point it seems you're intentionally trying to waste my time. I will remake the screenshot again for the 3rd time.
Is this different from MC-197201?

Yes this bug is completely different. That bug reports an intentional change to the dragon's behavior made in 1.9, a change that made it so the dragon would no longer charge the player from the air. This bug report also covers other aspects of the dragon's broken AI that that one does not.
Gotcha, thank you

Here are out Bug Tracker Guidelines: https://help.minecraft.net/hc/en-us/articles/360049840492-Mojang-Bug-Tracker-Guidelines
As you can see, the usage of any kind of game modification invalidates a report, no matter what the modification does. We do this to ensure that this is not just an issue with the specific mod.

Gotcha

Why is this now invalid again?

@unknown marked it as invalid probably because it's a feature request or not a valid bug

But it isn't a feature request, and it absolutely is a valid bug, a major one at that. Please elaborate on your reasoning.

Why was this marked as invalid? This is a major bug that has been in the game for way longer than it should have been. Many others experience this same issue. Ex: MC-201944

Please reopen this issue, as no reason was given for it to be classed as "Invalid". For the better of the game, this should be reopened.

I can confirm now after extensive testing that this bug appeared FIRST in 19w08b
This is actually a Bug, I compare both Bedrock and Java Dragons... and because of this bug the "Dive Attack" of the dragon is missing in Java.

I get the feeling this bug was marked as Invalid because it'd be difficult for the devs to fix (this does not excuse it though, because it's a major bug and makes the fight way less exciting).

Reiterating, should be reopened. Reason not given for this to be closed, bug remains.

This issue is not invalid: the Ender Dragon has incorrect vertical velocity (roughly a tenth of what's its supposed have), causing most of the AI issues described here.
This issue should be reopened.

Confirmed in 1.20.6.
Please provide a screenshot of your game with the F3 debug screen enabled while looking at an affected ender dragon.