When you have a half slab placed on its top half ( with the "y" value as an integer ), and you use flint and steel or a command to place fire on top of the slab, the fire will be invisible. This creates an invisible light source but will still emit particles and sounds.
Thank you for your report!
However, this issue is a Duplicate of MCPE-44395.
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:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki