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.

Original Description:
When snowlogging ground cover you can place the snow top either by placing it while looking at it or on the corners but with flowers, mushrooms you only can place it on the corners to snowlog.