Cannot pour out water on doubled slab

Thank you for your report!
However, this issue is a Duplicate of MC-126906.

If you have additional information, please add it to that report.
Please search before reporting, as it's likely that one exists already.

Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki

Actually, i found that if you place the slab on the "type: top". You can place the water, but then that water is inside that block, not on the top.

Maybe that will help.
About searching, i had to miss that issue 😃. Sorry!