Birch forests, old growth birch forests, and meadows generate wildflowers naturally. Using bone meal on grass blocks in a birch forest or old growth birch forest has a chance to generate wildflowers, but attempting to do the same in meadows will only result in short/tall grass and gradient flowers, wildflowers will not generate.

How to reproduce

• Generate a world with the seed 12333
• You should spawn right next to a meadow biome with naturally generated wildflowers
• Use bone meal on grass blocks within the meadow biome
• Bone meal generates short grass, tall grass and the gradient-dependent flowers (in this case dandelions and azure bluets), but wildflowers never generate

Related issue with bone meal and flower generation: MC-268689