When a moss block is bonemealed and it spreads to lower layers of blocks, they require multiple air blocks above to be valid.
When bonemealing the center moss block, the moss can spread to both lower stone blocks and convert them:
→
[media]If we add obstructing blocks like this, it can't spread downwards anymore:
[media]
Thank you for your report!
We're tracking this issue in MC-224615, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki