The sifted gravel from Modest Flint Overhaul won't crush with the hammer into sand. This is a problem because any stone that crushes into gravel turns into this kind of gravel, and any gravel mined also turns into this kind of gravel. Regular gravel in the world from vanilla Minecraft generation doesn't have this problem, but it has the curious result of producing sand, sifted gravel, and flint, all at once.

Weird mod compat issue. In the meantime, you can add a recipe for it.

How do I do that? I mean, is there a config, or do I need to add a recipe json somewhere?

You can add your own recipes with the data pack json

Weird mod compat issue. In the meantime, you can add a recipe for it.

Please report the issue to the author of Modest Flint Overhaul. Tell him his Loot Modifier is conflicting with mine.