Summary:
You cannot snow non-replaceable blocks by aiming at them. This applies to all flowers, but does not apply to grass, fern, warped roots, crimson roots, nether sprouts.

Steps to reproduce:

1. Place any flower.
2. Take a snow layer.
3. Aim at the flower and try placing a snow layer.
4. Aim at the block on which the flower is placed.
5. Try placing a snow layer.

1. Place any grass, fern, nether sprouts, warped roots or crimson roots.
2. Take a snow layer.
3. Aim at the grass, fern, nether sprouts, warped roots or crimson roots and try placing a snow layer.
4. Aim at the block on which the grass, fern, nether sprouts, warped roots or crimson roots is placed.
5. Try placing a snow layer.

Observed results:
You cannot place a snow layer while aiming directly at a flower.

Expected results:
You can place the snow layer by aiming directly at the flower.