Welcome 234

As of 21w15a, worlds from these snapshots seem to blend so I don't know it was accidentally fixed.

Sometimes it can cause a crash when trying to spawn. Don't know if that is related to this issue

Can confirm, attached crash report and log error is below

Note: This happened when I was playing around in my snapshot world, the warden was on lava and I was spawning a lot of frogs inside a cave at Y -40.

[21:52:41] [Server thread/ERROR]: Encountered an unexpected exception
z: Ticking entity
	at net.minecraft.server.MinecraftServer.b(SourceFile:907) ~[22w12a.jar:?]
	at net.minecraft.server.MinecraftServer.a(SourceFile:847) ~[22w12a.jar:?]
	at fhk.a(SourceFile:97) ~[22w12a.jar:?]
	at net.minecraft.server.MinecraftServer.w(SourceFile:693) ~[22w12a.jar:?]
	at net.minecraft.server.MinecraftServer.a(SourceFile:271) ~[22w12a.jar:?]
	at java.lang.Thread.run(Thread.java:833) [?:?]
Caused by: java.lang.ArrayIndexOutOfBoundsException: Index 24 out of bounds for length 17
	at it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap$KeySet.forEach(Object2IntOpenHashMap.java:1201) ~[fastutil-8.5.6.jar:?]
	at bpg.a(SourceFile:32) ~[22w12a.jar:?]
	at bpm.R(SourceFile:344) ~[22w12a.jar:?]
	at ayx.eH(SourceFile:761) ~[22w12a.jar:?]
	at ayv.w_(SourceFile:2619) ~[22w12a.jar:?]
	at ayx.w_(SourceFile:548) ~[22w12a.jar:?]
	at bnr.w_(SourceFile:43) ~[22w12a.jar:?]
	at ayv.k(SourceFile:2364) ~[22w12a.jar:?]
	at ayx.k(SourceFile:348) ~[22w12a.jar:?]
	at bpm.k(SourceFile:303) ~[22w12a.jar:?]
	at aef.a(SourceFile:705) ~[22w12a.jar:?]
	at ccx.a(SourceFile:450) ~[22w12a.jar:?]
	at aef.a(SourceFile:366) ~[22w12a.jar:?]
	at cvc.a(SourceFile:54) ~[22w12a.jar:?]
	at aef.a(SourceFile:334) ~[22w12a.jar:?]
	at net.minecraft.server.MinecraftServer.b(SourceFile:903) ~[22w12a.jar:?]
	... 5 more

Can confirm still in Pre release 7

Did you load this world before snapshot 21w42a?

Can confirm

This is also making sharp borders in the caves from a Pre-release 5 world

It's rare to find lava in these type of caves now

I think it's intentional as moving the Super flat down can cause problems with custom maps.

Update on this issue

After testing around a bit more on this issue, I've seen that these missing block chunks before upgrading to 1.18 have no bedrock layer at the bottom of the chunk (This seems to only happen on edge where a chunk is still not completely generated yet).
The cause of this issue is when caves are generated below old chunks, the chunk contains no bedrock so it won't attempt to place caves below this chunk since after testing around, the chunk must contain at least 1 block of bedrock at Y=0 in the world for it to fill the bottom with caves. The fix would be detecting if the chunk contains an block at Y=0 instead of detecting only Bedrock.

Here is an example of how this bug happens and what I was talking about above:

Intended, since this used to generate on older version before 1.17 but seems more common in these pictures.

Can confirm
Pre-release 5

World with 3 chunks with air below 0

Download:

Coordinates: /tp -1201 -9 -85
Tested on Pre-5

After flying around in Pre 4 and 5 I was no longer getting lag spikes which means this might be fixed.

A fix for alpha holes is you can place below 0 caves if there is a block that isn't equal to air blocks or other invisible blocks like barriers too or glass. But if there is stone, bedrock or something it will fill below 0.

Edit: After testing around I noticed the thing I sent above is the case now as I saw it only has the columns down to 0 if the block is air as there was lava with no columns of air below it, so this should be kept marked to fixed

New Fixed world download: https://easyupload.io/1313ow
Same coordinates: /tp -853 -14 24

After teleporting, check if there are missing blocks below this coordinate.

Still can reproduce on Pre release 3

Can no longer reproduce, I explored my very large world and seems to be no errors related to this bug.

Yes, seen it occur on my other old world

I'll check again to confirm it's still reproducable

Having the same issues again in another world but without the console errors this time

Version is pre release 2

Can confirm, I keep getting constant lag spikes while flying around, mostly in old chunks.

Appears to be fixed in Pre release 2

Relates to MC-240030 which has been Fixed for a Future release