When creating a dimension datapack, and defining a dimension with a set of biomes (as in example files attached), only the first biome in the set is used.  In the attached example, since the first biome listed is "minecraft:beach", the only biome in the entire dimension are beaches.  None of the other biomes lists are created.  If the entry for "minecraft:river" where to be swapped with the entry for "minecraft:beach" (and therefore the river entry would be first), the only biome in the entire dimension would be rivers.