The bug
Enderman spawn eggs in the form of an item entity or in an item frame will cause the chunk its in to regenerate when upgrading from 1.12.2. Enderman spawn eggs in containers such as chests are not affected. Other spawn eggs also do not cause this issue.
This issue is not world specific, but a world from 1.12.2 is provided:
Exception
Partially omitted due to length
1.13-pre5\: [^launcher_log.txt]
[0630/195720:INFO:GameCallbacks.cpp(188)] game/com.mojang.datafixers.DataFixerUpper (Server thread) error Something went wrong upgrading!
java.lang.IllegalStateException: Named type name doesn't match: expected: block_name, got: block_state
at com.mojang.datafixers.types.templates.Named$NamedType.write(Named.java:146)
...
at com.mojang.datafixers.DataFixerUpper.update(DataFixerUpper.java:78)
at hj.a(SourceFile:493)
at bmt.b(SourceFile:98)
at bmt.a(SourceFile:84)
at bmt.a(SourceFile:120)
at sw.g(SourceFile:92)
at sw.b(SourceFile:114)
at sw.a(SourceFile:150)
at net.minecraft.server.MinecraftServer.g_(SourceFile:385)
at dej.a(SourceFile:116)
at dej.d(SourceFile:132)
at net.minecraft.server.MinecraftServer.run(SourceFile:505)
at java.lang.Thread.run(Thread.java:745)
[0630/195720:INFO:GameCallbacks.cpp(188)] game/bmt (Server thread) error Chunk file at 15,14 is missing level data, skippingAttachments
Comments 4
First of all, I don't know what spawn egg it was,
second, it was inside the chunk,
third yes, and
fourth, no.
I only know it wasn't any of the following spawn eggs:
Horse, Sheep, Llama, Chicken, Pig, Cow, Ocelot, Wolf, and Parrot, as those also exist in that chunk, but they don't cause the reset.
However, there were some others that are in an adjacent chunk but don't cause this effect in that chunk, those include
Creeper, Skeleton, Zombie, Spider, Endermite, Guardian, Shulker, Polar bear, Wither Skeleton, Stray, Husk, and the formally existent undefined spawn egg.
Other than that, it would be down to any of the 1.12.2 spawn eggs that were not listed above.
My advice, try loading up my world save, (I know you have it, I sent you multiple bug reports with it in them), in 1.12.2, and assigned all the still possible spawn eggs to individual chunks, (try placing four of them around a block suspended by a fence post), and check to see which one disappears when upgrading to 1.13 pre-release 5, (if using the method above, then load up in 1.12.2 and upgrade again, but only with the four spawn eggs in item frames as test subjects, each in it's own chunk).
Hope this helps. Rockatoa, Brickticks out!
Mod edit:Formatted wall of text
Cannot confirm.
What spawn egg was it?
Where was the item frame located (ex. inside the chunk, on the chunk border)?
Can you still reproduce this issue in 1.13-pre5?
Does this occur in 1.12.2?