On the seed 3737625602597677918, at the rough coordinates 1000,1000 is an absolutely massive sized mushroom island. I have attached a video of a quick fly over in spectator mode to get the gist. Many other seeds that I have checked seem to also have fairly large mushroom islands (if not a tiny island), but none close to this. It just seems too much for what is supposed to be a more rare biome. This maybe a result of MC-236649, where the islands were to small. Feel free to tack this one onto it if it is the same issue.