The VRCDefaultWorldScene folder, it's contents, and the scene file are reset/re-added when opening up some worlds projects. This makes sense for the first time the project is opened but this can happen for some projects on every project open. Data loss can occur if the default scene was being used for storing world data.

It may be a race condition with loading assets.

This also appears to be common when working with a world shared via git, as the VRCDefaultWorldScene files are re-generated and marked as unnecessary changes in a commit.