Firstly I should say I have opened this bug before a few days and years ago. With all due respect, I'm requesting moderator Kumasasa to leave this one to another mod, because I'd like a second opinion. Contrary to what was thought before:

• Even when standing around for ten minutes to wait till there are 0 chunk updates AND when standing still every 100 blocks, the world still generates differently at times. So it's not a duplicate of MC-15820

-It is also not a duplicate of MC-15820 because the world does not generate in stripes at all. That bug was probably fixed long ago. This is a different issue, I swear.

But because there was much confusion on how to replicate the bug, it was eventually closed by the mods. Now I have a quick way to replicate it which is described at the end. I'm 100% positive this is a (minor) bug, so my request is to take it seriously!

A seed is supposed to generate exact the same world on all PC's. But some world chunks seem to have more versions. So most of the world is the same, but in 50% of all instances the seed is used to generate a new world, the seed is different. This is visible in the 2 screenshots I uploaded. There are two versions of this wall: one with coal in it, one with far less coal in it.

I discovered it this way: I was playing along with an LPer with the seed "PythonGB LP World". The world was the same, but upon seeing this wall, he had the version with many coal in it, I had just one coal spot. When playing along further, there were more instances (differently placed trees, pools, landscaping). This is kind of frustrating because when an LPer discoveres for instance a diamond chunk, it may not be there in my world. This happens with every seed. So every seed has some differently generated chunks.

How to replicate this:

1. Make a world (use the seed "PythonGB LP world" or "1862394111" for verification)
2. Go to the coördinates specified on the screen (make sure the world is loaded properly first just to make sure)
3. Observe the wall
4. Recreate the world, like ten times
5. Around 50% of the time, the wall looks different. This affects some other chunks as well and affects every seed.

Mr. Kumasasa tried this 5 times and every time he got one version. This is why I'm requesting a second opinion. I tried this at two PC's, both fast enough to run the game. If a second mod also cannot replicate this or says it's some duplicate, then I will give up on this.

This will work with every world and every seed, but you'll need to find a place that is "sensitive" to it. Some chunks always generate the same, like they're supposed to.

I didn't quit and reload during playing,
50% of the time (may be more or less, it's based on chance), the wall will be different. So a seed seems to have at least two versions of some chunks.