Mushroom fields biome generated in the middle of a deep ocean biome without any biome transition

I gave the latest snapshot a quick test run and came across this mushroom fields biome that is basically a table mountain in the middle of a deep ocean biome. Apart from a very steep coast at the border of the two biomes, there is no transition to speak of around the entire (quite large) island. I would have expected to see at least either a bit of the matching regular ocean variant or a mushroom fields shore somewhere.

Seed: -1721013089577705712
Coordinates: around -3100/-4800