Forests, birch forests, old growth birch forests, and dark forests generate lilies of the valley as part of the forest_flowers feature. Despite this, using bone meal on grass blocks in these biomes only generates dandelions and poppies, and no lilies of the valley.

This makes lily of the valley very difficult to farm, despite it being a relatively common flower, as currently it can only be regrown in specific areas in flower forests, depending on the flower gradient.

How to reproduce

• Generate a world with the seed 5946705697671253412
• Run the command /tp 45 64 0
• It's a forest biome with a naturally generated patch of lilies of the valley
• Use bone meal on the ground within the forest biome
• Dandelions and poppies appear, but lilies of the valley never generate

Possible fix
If I had to guess, bone meal only generates the default dandelions and poppies because forest_flowers also includes tall flowers. One possible fix would be removing the flower_default feature from the forest biomes, and instead using two separate features for small forest flowers (dandelion, poppy, lily of the valley), and tall forest flowers (rose bush, peony, lilac). Bone meal would be changed so that - in forest biomes - it generates flowers using the new "small forest flowers" feature. This would allow bone meal to generate all three small forest flowers.

Note
Flower forests also generate lilies of the valley, as part of the forest_flowers feature and the flower_flower_forest gradient. Using bone meal in flower forests applies the gradient, meaning lily of the valley can only be regrown in areas where the gradient allows it. I'm pointing this out because I imagine this behavior - in flower forests specifically - is intended, although it looks really inconsistent on a surface level.
Splitting forest flowers into two separate features for small flowers and tall flowers would allow flower forests to generate lilies only as part of the gradient - while keeping the tall forest flower feature - resolving the inconsistency described above.
Anyway, changes to the noise used to generate the gradient also made lilies of the valley considerably rarer than before. Issues with lily of the valley in flower forests are covered in MC-241620.