Blocks with no damage value variants obtained as items stack in the first available stack, even if they have a different damage value.

If you use a damage value of 1 first, they all stack using a missing model.

Steps to reproduce:

1. Give yourself a brown mushroom block using /give @p brown_mushroom_block 1 1
2. Notice it appears as a purple and black checkerboard.
3. Give yourself a brown mushroom block using /give @p brown_mushroom_block 1 0
4. Notice it stacks, and has the missing model
5. Clear your inventory
6. Give yourself a brown mushroom block using /give @p brown_mushroom_block 1 0
7. Notice it looks fine
8. Give yourself a brown mushroom block using /give @p brown_mushroom_block 1 1
9. Notice it stacks and looks fine