Builds from past 3 weeks disappeared when 19w40a downloaded

Can you attach the game logs from when the builds were last there please? You can find them in .minecraft/logs.

Thank you. I will try to get them this evening

Hi. My name is Leanne. I am Austin's parent. I have gone to launcher settings and turned on game logs, but I am unable to find the logs from 3 weeks ago

Can I get some help locating the logs if possible. 

Do Windows Key + R, type in %AppData%, and open the .minecraft folder. That's where the logs folder is.

This is Leanne again. Please be patient. I went through a great deal of the logs. The earliest warning threads date back to March 10, 2019 This thread has been present in all the logs from here onward.

[12:00:02] [Client thread/WARN]: Ambiguity between arguments [teleport, destination] and [teleport, targets] with inputs: [Player, 0123, @e, dd12be42-52a9-4a91-a8a1-11c01849e498]
[12:00:02] [Client thread/WARN]: Ambiguity between arguments [teleport, location] and [teleport, destination] with inputs: [0.1 -0.5 .9, 0 0 0]
[12:00:02] [Client thread/WARN]: Ambiguity between arguments [teleport, location] and [teleport, targets] with inputs: [0.1 -0.5 .9, 0 0 0]
[12:00:02] [Client thread/WARN]: Ambiguity between arguments [teleport, targets] and [teleport, destination] with inputs: [Player, 0123, dd12be42-52a9-4a91-a8a1-11c01849e498]
[12:00:02] [Client thread/WARN]: Ambiguity between arguments [teleport, targets, location] and [teleport, targets, destination] with inputs: [0.1 -0.5 .9, 0 0 0]

This warning thread has been present in every log from March 11, 2019 on

[15:57:29] [Client thread/WARN]: Skipping bad option: lastServer:

I have managed to narrow down at least logs that I believe were recorded at the time my son's builds disappeared.  I will be adding them as an attachment

I began seeing these warnings September 6, 2019[14:49:06] [Server thread/WARN]: Fetching packet for removed entity atx['Dirt'/1002, l='New World', x=-39.28, y=2.67, z=49.58] and on, as well as messages for [08:30:57] [Server-Worker-15/WARN]: Bytes mismatch 4864 0. There were so many I wasn't able to include them among the attachments at this time.

Hmm, thanks, that's quite a lot. Now, the logs are about multiple different worlds; which world did the issue occur in? Also, what exactly was disappearing, were therere certain square areas that seem to have reset, or did the inventory just get cleared?

Also, some logs are for 1.14.4 and some logs are for 19w41a. Did you upgrade any world from 1.14.4 to 19w41a and then had the issue occur in that world? If so, it would be great to have the backup of that world before it was upgraded. Also note that downgrading worlds from 19w41a (or any other snapshot) back to 1.14.4 will cause issues and is therefore not supported.

A bit of info on the warn messages: The "Ambiguity" warning messages appear always and for everyone, because the syntax of the /teleport command is not unambiguous currently. That's not relevant to this issue. "Skipping bad option" is similar, caused by a different bug, that's irrelevant as well. The 'fetching packet' one seems odd, but that was a month ago, so I'm not sure whether it might have something to do with this issue.

The inventory and builds from worlds "Blue Marble" and "My World" has been put back to what it looked like before the builds began. My son began building one of the fortress structures back in August I think. Everything was lost up to that point.  There was a large fortress made of iron blocks in "My World" and a 3 story build made from wood and glass in "Blue Marble"  There were storage chests and barrels in each that were full and are gone as well.  I am not expecting miracles, but I would like to prevent this from happening again as my son put a lot of work into the builds and accumulating storage items. He was pretty upset when things disappeared. We always back save and back up the world before upgrading.  "Fetching packet' has been found on multiple logs since September 6th as well as the Bytes mismatch.   

Sometimes the game creates backups (usually when changing versions), which you can find in .minecraft/backups. Usually backups need to created manually though.

It is known that 1.14.4 occasionally can corrupt worlds, but that issue has been supposedly fixed in the 1.15 development phase. I'll notify the responsible dev that the issue is still persisting.

My son has recently rebuilt his world. However, we logged on today and it has crashed again. The whole world has been rearranged.  I have a copy of the log.

"Latest Crash"  I will look for the backup.

What need to be done after I find a backup. Thanks again for your patience with this issue

Once you find a backup in the backups folder you could either duplicate it and put it in the .saves folder... Just make sure you uncompress/extract the folder. Once you have done this just open minecraft and it should be back in the singleplayer worlds part of the game. If you have trouble finding these folders one thing i do that makes things easier is on the title screen of the game go to options then click on resource packs. There will be a button that says open resource pack folder. You can click on that and then at the top where it tells you all the folders you are in click on the .minecraft folder. Then from there your folders will all be right there. C:\Users\"USER"\AppData\Roaming\.minecraft\resourcepacks. (Note: USER in all caps is a filler for your computers username. whoever is logged in will show there.)

You've deleted a few files. Can you attach them again please? They'll be very useful when trying to find a solution for the issue.

[10:04:22] [Server thread/ERROR]: Exception loading structure piece with id minecraft:nvi
java.lang.NullPointerException: null
	at ccq.<init>(SourceFile:28) ~[19w41a.jar:?]
	at ccq.a(SourceFile:53) ~[19w41a.jar:?]
	at chs.<init>(SourceFile:44) ~[19w41a.jar:?]
	at chy$$Lambda$1139/1717737290.deserialize(Unknown Source) ~[?:?]
	at abf.a(SourceFile:18) ~[19w41a.jar:?]
	at clb.<init>(SourceFile:48) ~[19w41a.jar:?]
	at cfe$a.<init>(SourceFile:35) ~[19w41a.jar:?]
	at cez$$Lambda$1075/1234453970.load(Unknown Source) ~[?:?]
	at clf.a(SourceFile:80) [19w41a.jar:?]
	at cam.a(SourceFile:419) [19w41a.jar:?]
	at cam.a(SourceFile:175) [19w41a.jar:?]
	at wm.j(SourceFile:511) [19w41a.jar:?]
	at wm$$Lambda$2866/154086159.get(Unknown Source) [19w41a.jar:?]
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1582) [?:1.8.0_51]
	at aik.h(SourceFile:144) [19w41a.jar:?]
	at aik.v(SourceFile:118) [19w41a.jar:?]
	at wy$a.v(SourceFile:550) [19w41a.jar:?]
	at wy.d(SourceFile:274) [19w41a.jar:?]
	at net.minecraft.server.MinecraftServer.aZ(SourceFile:726) [19w41a.jar:?]
	at net.minecraft.server.MinecraftServer.v(SourceFile:714) [19w41a.jar:?]
	at aik.c(SourceFile:127) [19w41a.jar:?]
	at net.minecraft.server.MinecraftServer.u(SourceFile:699) [19w41a.jar:?]
	at net.minecraft.server.MinecraftServer.run(SourceFile:654) [19w41a.jar:?]
	at java.lang.Thread.run(Thread.java:745) [?:1.8.0_51]

in Latest Crash.txt. Might be same as MC-163477

This ticket has been merged into MC-161823.