Grass (the decorational X-shaped block) can be found floating, especially over water.

decorational grass blocks, and occasionally flowers, can be seen floating over the edges of small ponds around the world. the block underneath them is dirt, as if the grass block had just vanished, leaving a one block gap of air in between said dirt block and decorational grass block. placing a block that supports plant life underneath the grass will not break the decorational grass, however, this is not true for a block that does not support plant life, such as obsidian.

It should be mentioned that a block update on the side of the decorational grass will break it.

Mava was out of date, but updating it did nothing when reopening the game fully.

My seed was 9003069667693158656, and one screenshot contains coordinates of where an instance of the issue can bee seen.