Upgrading custom world to 21w43a ignores min_y and height

The bug

If you open this custom world

in 21w43a after you generated it in 21w42a results in all terrain below -64 disappearing and any blocks being placed above 320 / max_y.

How to reproduce

1. Open Minecraft in 21w42a

2. Create a world using the custom world settings in

1. ✔ Enter spectator mode, and observe that the world bottom is lower than Y=-64 and world top is higher than Y=320 (e.g. by placing a block above it)

1. Open Minecraft in 21w43a

2. Play the world you created previously to upgrade it
   ❌ The terrain previously generated below Y=-64 has been removed, and the bottom of the world is now found at Y=-64.
   ❌ Blocks above Y=320 are removed, and the top of the world is now found at Y=320

Expected behaviour

1. The upgraded world would not have all terrain below Y=-64 and above Y=320 removed, the dimension's bounding boxes / min_y and height should stay unchanged.

Error

Couldn't load chunk [11, -4]
java.lang.ArrayIndexOutOfBoundsException: Index 58 out of bounds for length 24
	at cow.a(SourceFile:350)
	at cow.a(SourceFile:290)
	at cps.a(SourceFile:202)
	at abu.m(SourceFile:579)
	at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1764)
	at atg.c(SourceFile:151)
	at acf$b.c(SourceFile:534)
	at atg.y(SourceFile:125)
	at acf$b.y(SourceFile:543)
	at acf.d(SourceFile:278)
	at net.minecraft.server.MinecraftServer.be(SourceFile:763)
	at net.minecraft.server.MinecraftServer.y(SourceFile:751)
	at atg.c(SourceFile:134)
	at net.minecraft.server.MinecraftServer.x(SourceFile:736)
	at net.minecraft.server.MinecraftServer.b(SourceFile:497)
	at net.minecraft.server.MinecraftServer.f_(SourceFile:329)
	at fcg.e(SourceFile:72)
	at net.minecraft.server.MinecraftServer.w(SourceFile:655)
	at net.minecraft.server.MinecraftServer.a(SourceFile:269)
	at java.base/java.lang.Thread.run(Thread.java:831)