The Bug
When you bone meal a moss block on a replaceable block like stone or deepslate, it spreads around. However, if the replaceable block has any transparent, non-full block on top of them, the moss does not spread.

Step to Reproduce
1. Place a moss block inside stones/deepslates
2. Place down blocks like torches, fences, iron bars, or chains.
3. Bone meal the moss
---> It spreads, except the ones that have transparent blocks above it.