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:
Give yourself a brown mushroom block using
/give @p brown_mushroom_block 1 1
Notice it appears as a purple and black checkerboard.
Give yourself a brown mushroom block using
/give @p brown_mushroom_block 1 0
Notice it stacks, and has the missing model
Clear your inventory
Give yourself a brown mushroom block using
/give @p brown_mushroom_block 1 0
Notice it looks fine
Give yourself a brown mushroom block using
/give @p brown_mushroom_block 1 1
Notice it stacks and looks fine
Linked issues
relates to 1
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Comments 5

sounds intended, as it is not a supported data value, and you aren't supposed to get them in your inventory

Relates to: MC-57332

@FVbico
Yes, it is likely caused by an unsupported value, and I would not argue with a resolution of either "Works as intended" or "Won't fix" from Mojang. The main thing is the behavior is inconsistent with other items, allowing incorrect damage values, but still stacking them with correct ones when obtained as items.
Relates to MC-47596.
Confirmed. When you type
/give @p brown_mushroom_block 1 1
, notice that the missing texture item flashes briefly in front of your face.