The bug

Warped and Crimson stems use the same map color regardless of whether they are oriented horizontally or vertically.
This is inconsistent with other log types, such as birch logs.

Vertically oriented logs should have the same color as the respective hyphae blocks (see also MC-165776).

How to reproduce

1. /fill ~1 ~ ~1 ~6 ~ ~6 minecraft:warped_stem[axis=y]
   

2. /fill ~7 ~ ~7 ~12 ~ ~12 minecraft:warped_stem[axis=x]
   

3. Create a map
   The blocks have the same colors on the map despite them having different top textures