Despite the flattening of Mushroom Stems into their own distinct block, mushroom_stem, red_mushroom_block ["huge_mushroom_bits"=15] and brown_mushroom_block ["huge_mushroom_bits"=15] still exist as distinct blocks.

This can be seen by how doing /setblock [x y z] red_mushroom_block ["huge_mushroom_bits"=15] followed by /testforblock [x y z] mushroom_stem shows that the game views the block placed as a Red Mushroom Block, and not a Mushroom Stem. The same applies for Brown Mushroom Block with the state set to 15. Per how blocks have been flattened, using the old syntax should convert them to mushroom_stem.

In addition, the new mushroom_stem block has an unused huge_mushroom_bits block state that serves no function, yet continues to not just be allowed in commands without the game throwing an error, but in fact be suggested after typing in /setblock [x y z] mushroom_stem [ into the text box.