Use a separate node for General Height, and use it in each biome with a Function noide
So, if you have a biome with white mask on top it will completely remove all the previous biomes. This is what probably happens with your ocean biome. It's not possible to read the height for applying biome (to fill only lowlands with sea) - just because biome can change the height, and after the height is changed all other biomes should be re-applied. This will cause an infinite loop. The basic concept is that biomes are independent graphs, that work on their own if being applied in MM directly, without Biomes Set. They can share same base height nodes, it can be even brought to Function node to make it instance, but they can't receive information from other biomes.