FireSkorpion

Update: The Endermen teleport to any movement blocking material, even if the water is accessible from the top (like waterlogged bottom slabs, or waterlogged trapdoors(on the lower block) and things. https://youtu.be/RyI8t3DV0Hc

This confirms that the issue has to do with the tags different blocks have (movement-blocking, waterlogged, etc.),

• and there needs to be either a new tag created, that classes blocks if the endermen can stand on the top safely,
  OR

• the Enderman AI has to also check if the block it will teleport to is waterlogged (of course the simpler approach, and more desirable)

I would side with @Loak van Vucht

This report is invalid in my opinion, otherwise the non- waterloggable doors would be a bug too

@Hudson Schlumberger
That makes sense, and that means that there just needs to be some blocks above, that aren't valid for teleporting, like what the water does in how I experienced it
It might not even need to block the teleporting, it will just be less likely and harder to spot, because the endermen would directly teleport to the 'ceiling' blocks

can confirm for non-optifine 1.16.5  

@Adan Chappuis Ideally remove the 'optifine' bit

"In addition, this is different from the standard weight of 5 in (most) ocean type biomes."
This part is apparently intended though, to combat the issue of there being significantly less spawning spaces in rivers, compared to oceans.
 

May I suggest for the title to include 'horizon' or 'black horizon'?
I was just about to report the issue, as I didn't find this ticket before, and was pointed here by a helper on the discord!

@Micheal Wobst
In the most recent snapshots this problem has been reintroduced.
Because the build limit has been extended down, while you are in a flat world, you are below y level 0, which means you get a black horizon if you fly up a few dozen blocks up from y level -64. (as shown in the screenshot)

I am not sure whether to open a new report, that has not been marked as 'Resolved' yet, because this report is also very old, and probably has a different cause.

For everyone that is interested in this issue, it is a duplicate and I would encourage you to vote for the original bug report! 😉

@Mark Derickson I just came across MC-197647 that is the reason why I came to check on this page again.
My report seems to dublicate it, but I find my title a bit easier to understand.

Edit: This is also the reason why I didn't find MC-197647 in the first place.

The fix of this has led to players not being able to step down a block while sneaking with a block above the head (MC-209925).
You now need to mine the two blocks above, and to the side of the player to jump a full block up and then fall two, onto the block you wanted to get to.

It probably conflicts with the fix of this bug and will therefore probably from now on be seen as intended behaviour.