Possibly related to: MC-214793. (Please read entire report before marking a duplicate)

The bug

I generated a new world with the same seed as my main survival world in 21w15a. I flew around a bit to see how different world generation is in 21w15a vs 21w05b. I soon saw that there were a ton of creepers spawning, some zombies and skeletons that were burning, and a few spiders, even though it was broad daylight. Some skeletons and zombies in the water. (The world was fundamentally broken in other ways in this area, as well, such as a glitched out ruined portal, partial trees, and caves with glitched out generation.) The sky light says that it was light level 15 also, so the chunks were lit normally, making this a distinct bug from MC-214793.

Note that it only happened once. I loaded a new world with the same seed, and all the glitched out structures were normal, and the mobs did not spawn. In addition, the mobs stopped spawning once I reloaded the world.

Info from duplicate reports: Sometimes the zombies and skeletons burn immediately after spawning, and sometimes they don't burn at all. 

Crash report (this was a forced crash, the game did not crash on its own):

---- Minecraft Crash Report -------- Minecraft Crash Report ----// I let you down. Sorry
Time: 4/15/21 12:11 AMDescription: Manually triggered debug crash
java.lang.Throwable at dul.b(SourceFile:475) at dun.q(SourceFile:1668) at dun.e(SourceFile:1042) at dun.e(SourceFile:700) at net.minecraft.client.main.Main.main(SourceFile:217)

A detailed walkthrough of the error, its code path and all known details is as follows:---------------------------------------------------------------------------------------
– Head --Thread: Render threadStacktrace: at dul.b(SourceFile:475)
– Affected level --Details: All players: 1 total; [ekw['PlatinumMine'/46625, l='ClientLevel', x=-352.71, y=70.51, z=-305.08]] Chunk stats: 1681, 1296 Level dimension: minecraft:overworld Level spawn location: World: (128,66,14), Section: (at 0,2,14 in 8,4,0; chunk contains blocks 128,0,0 to 143,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511) Level time: 5282 game time, 5282 day time Server brand: vanilla Server type: Integrated singleplayer serverStacktrace: at ehy.a(SourceFile:370) at dun.c(SourceFile:2259) at dun.e(SourceFile:719) at net.minecraft.client.main.Main.main(SourceFile:217)
– System Details --Details: Minecraft Version: 21w15a Minecraft Version ID: 21w15a Operating System: Windows 10 (amd64) version 10.0 Java Version: 1.8.0_51, Oracle Corporation Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation Memory: 1216994440 bytes (1160 MB) / 2147483648 bytes (2048 MB) up to 2147483648 bytes (2048 MB) CPUs: 8 JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M Launched Version: 21w15a Backend library: LWJGL version 3.2.2 build 10 Backend API: GeForce GT 530/PCIe/SSE2 GL version 3.2.0 NVIDIA 388.73, NVIDIA Corporation GL Caps: Using framebuffer using OpenGL 3.2 Using VBOs: Yes Is Modded: Probably not. Jar signature remains and client brand is untouched. Type: Client (map_client.txt) Graphics mode: fancy Resource Packs: vanilla Current Language: English (United Kingdom) CPU: 8x Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz