Describe the bug you're experiencing

I join a world, add stuff from the MTR mod, rejoin world and all the rails and other stuff that should have saved isn't there anymore. I disable your mod and retry this and the objects remain.

Reproducability

Install both the MTR mod, the one i used was version 3.2.2 Hotfix 2, and the latest version of your mod for 1.20.1.

Mod up to date

• Before submitting this issue I updated to the newest version and reproduced it

Minecraft version

1.20.1

Modloader version

Forge

Logs

https://gist.github.com/WalterLezerus/8627e52a955656c48f6a95e8a1a9bba3

works just fine for me, are you sure you mean the "chunksending" mod? if so it might be a correlation <-> causation switchup

I do mean the chunksending mod. While it was disabled, I was able to save the world properly. I rejoined to make sure it had the stuff still there. Then I left, closed minecraft enabled your mod and rejoined left and joined again and it was all gone. it allowed me to place stuff while the world was up but once it restarted it all vanished.

do you maybe mean a different mod than chunksending? Chunksending only alters the rate at which data is sent to the client, it has no relation to data saving

No, it was definitely your mod that had issues. We tested disabling different mods and loading the rejoining a world but once we disabled your mod and only your mod did it actually save the data from the train mod.