I'm having issues using the forest_rock configured feature.

I'm having issues getting forest_rock configured features to generate on anything other than dirt and grass. I've created my own configured feature of type "forest_rock", and created a placed feature file which should be able to place it programmatically, but it doesn't generate in the world, and I couldn't even place it with the /place command. I think I've confirmed it's an issue with the forest_rock feature type specifically because leaving everything else the same but replacing the forest_rock feature with a simple_block feature works just fine.
I figured out after some trial and error that it will only generate on top of a grass block, but I didn't have any restriction in the placed feature file. I even set up a block_predicate_filter in my placed feature file to explicitly match red sand (the target biome is a red sandy desert), in case there's an implicit definition of that that I might override; that also did nothing. Is this something that's hardcoded? I haven't found any references to it being hardcoded to generate on grass.
I'm just starting to learn about world gen features, so I'm hoping I've just missed something obvious to a more experienced person. Any advice would be appreciated! Thanks! 🙂