The texture of a block with fully grown kelp right below isn't being displayed on a map - the map acts as if there is no block there at all, rendering only the kelp stalk.

Steps to reproduce:

1. Find kelp stalk close to sea level, or build a stalk up to one block below sea level.
2. Place a block in the ocean, at sea level, on top of the stalk.
3. Use a map to render the area (it does not matter if this is before or after step 2).

Although this occurred on a server, I was able to reproduce the bug with a freshly generated world in singleplayer.

As can be seen in the first attachment (2018-06-29_22.26.26), the wool texture is not rendered, but the shape of the kelp underneath is. I have also attached a photo of the map art I discovered this bug with, so you can see how it looks on maps with more afflicted blocks.

This did not occur in pre4.