Water visible through waterlogged upper slab when transparent block placed on top

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.