The bug
While trying to reproduce a bug for another bug report I haven't made yet I have discovered a weird occurrence. Every time I generate this world with the same settings it looks slightly different.
Edit: This doesn't only affect this world, this affects every world and it also affects every chunk of every world.
Basically the lush caves decorator isn't seed related resulting in you getting different world every time.
How to reproduce
Create a singlebiome lush caves world with seed -8508883206669806165
Go to: /execute in minecraft:overworld run tp @s -26.34 -36.86 171.23 -482.09 41.75
The area looks like this:
Now exit to menu and recreate the world without changing any settings
The area now looks like this:
Repeat the step from Step 1 (create a new world with lush caves single biome and the same seed)
The area now looks like this:
[media]Now recreate this world
This time the area looks like this:
As you can see each time this area looks very different.
It's so inconsistent I cannot predict if you will be able to reproduce exactly the same worlds I did
Basically only the shape of the cave stays the same and the decoration generation is entirely different each time.
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Nope that affect only certain chunks.
Here every single chunk is affected but only affects the lush caves biome and is much more noticable.
It also happens in all seeds.
I wouldn't say its a duplicate but it is related

Can confirm. Ran into this while trying to reproduce other issues.

Can confirm in 1.17.1 Release Candidate 2, using a single biome world. Used the same seed, but a different command:
/execute in minecraft:overworld run tp @s -150.06 33.22 72.36 -102.09 41.75
due to the reverted change to world height.

Can also confirm this still happens as of 1.18 Exp-snapshot 3

@unknown Thanks, but the 1.18 experimental snapshots are not supported in the bug tracker.

Can confirm in 21w37a.
This also happens with large azalea trees, which can negatively impact gameplay by changing where you can locate a lush cave on the same seed.

Can confirm in 21w38a

Can confirm in 21w39a.

Dupe of MC-55596

This is NOT a dupe of MC-55596, this is caused by something different also this causes the entire lush caves to be different not just a few select chunks

If anything, this is a duplicate of MC-238073.

I've now re-resolved this as a duplicate of MC-238073. As far as I can tell it should be the same issue, but the impact is bigger in 21w37a. And since MC-238073 got higher priority it should be the parent ticket from now on.
For future reference, see also reddit discussion https://www.reddit.com/r/Mojira/comments/q23as5/mc238073_and_mc219200_should_be_reopenedwere/
Seems like a duplicate of MC-55596.