Use the datapack, create a new Buffet world, use the minecraft:birch_flower_forest and see.
Not only it lags the world for some reason, it also calls the biome an ocean.
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is duplicated by 2
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It might be a different issue then, I also came across this when I tried to use a custom biome as a buffet world but I thought it was [MC-197616].
Most likely a duplicate of MC-197616