Grass generation not moving with sea level in customised world generation

When creating a customised world, lowering the sea level does not affect the grass line, which remains at y=63. Below this point dirt is generated in place of grass. Below y=54 gravel is generated in place of grass. This means that creating a world with a very low sea to make use of the full height of the map for terrain creates grassless valleys. I have not yet checked if it also affects passive mob spawns.

(Note: I am sure that in some cases this may be desirable, but there is no option for moving the grassline independently either, as with ores. Could we either have one, or link grass to the sea level option please?)

Here's a screenshot, and the associated preset. The seed is 1.

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