To reproduce (gif attached for more info):

• put any slab block exactly one block from the floor at the bottom part of second block;
• surround it with any block in each direction;
• fill leftover half with water;
• break one of the 'wall' and let the water flow out;
• restore the broken wall
  

Result:
Water is still floating out of the limited space.

Expected:
Water is not floating out after the wall is restored.

Note: if you build the exact same structure at more than 1 block height, it works fine: