I believe this bug has been around for a long time, but basically grass just naturally generates mid-air, often over ponds of water. It's not a common issue, but I've still seen it a few times.

This is in a newly generated world from Snapshot 19w44a. I have experienced this on a previous server of mine generated on Minecraft 1.14.2.

Seed: -3490738132142550619

Location: /tp 33 73 -121