The objective was to use the /fill command to create a square island of grass blocks in place of the top level of the water.

I received a message saying the blocks were successfully filled. Visually, it looked as if the water blocks were filled with air blocks, creating a void that was refilled by water.

I do know the grass blocks where created because some grass seeds where floating in the water.

Using the fill command with other blocks (stone, logs, fences, leaves) all worked correctly.

Command: /fill 641 62 3275 686 62 3325 minecraft:grass

Seed: 6737968182046887777

add: Further use shows that filling with grass blocks with different shapes and nearby coords still results in grass blocks breaking once created.