I use a simple animation that turns a floor collider off when the Grounded parameter is false. Several days ago, I noticed that floor colliders on all of my avatars would randomly be off when they should be on. This happens in all worlds, but the video below shows the only reliable way I could reproduce it: moving onto right far side of the couch in Exoplanet Journey. The floor collider is sometimes off even when Grounded is true.

Avatar in video: https://vrchat.com/home/avatar/avtr_218cbc7c-62e7-49bb-aab6-a47b7bfe9afa

You can check the debug screen to verify that the animation is playing, but the floor collider is still missing.