Have you try using bucket or placing block around the grass block? I think it water source, not flowing water. Because flowing water has 7 blocks length, but at the picture is 8 blocks.
This is duplicate of MC-165173
Affects version 19w46b
It not bug, right-click at item frame to rotate the map
Confirmed for 19w46b
user-40ab1
Have you try using bucket or placing block around the grass block? I think it water source, not flowing water. Because flowing water has 7 blocks length, but at the picture is 8 blocks.