UPD: Forgot to mentions.
Minecraft: 1.16.1
Fabric Loader: 0.9.0 build 204
Fabric Api: 0.16.0 build 384

Step to reproduce:

1. Create new world with imported settings (e.g.) in vanilla Minecraft or with Fabric
   [Create New World -> More World Options... -> Import Settings -> Choose file with JSON-content from url]
2. Save and quit.
3. Try to open a world.

Expected Behavior: world open.
Actual behavior: world doesn't open.

Vanilla Minecraft instead can open this world and all functioned normally.

Log File:
Fabric Modded Log.txt // https://paste.ee/p/xexhf
Vanilla log file:
Vanilla Log.txt // https://paste.ee/p/24hmO
World with settings from URL:
New World.zip

this is the oldest bug still open. Is this fixed?

Ok registry sync, you've constantly proven yourself more problematic than useful