I am surprised by this visual effect. Possibly a bug?
To reproduce: Dig a hole in farmland (see picture). Pour water in hole. Put upper slab on top half of waterfilled hole (so the slab become waterlogged). Place a button on top of the slab.
Expected result: The upper slab would continue having its own color.
Observed result: When the utton is placed, the slab changes texture into water.
Note that making a dark oak upperslab and placing that on the water, and putting a dark oak button on top of that slab does not produce similar results.
Related issues
Attachments
Comments


Thank you for your report!
However, this issue has been temporarily closed as Awaiting Response
Is this still an issue in the latest version? If so, can you please add it to the affected versions (or mention it if you are not the reporter).
This ticket will automatically reopen when you reply.
Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki

Confirmed on Windows 10. Placing glass blocks or buttons on top of a waterlogged top slab surrounded by solid blocks produces the bug.
Update: this was already reported at MCPE-90780, so resolving as a duplicate.