This is a very good question and is precisely why we added "If the loop cannot travel in the wind’s direction upon entering a cell in which the wind blows (such as it results in the loop hitting the edge of the grid, or a black cell), it may not enter that cell" to the new version of the IB (see download link above).

In the example, if the loop travels down from R2C4 to R3C4 it would be hit by the wind and will not be able to travel in the wind's direction as it will crash to the edge of the grid.
Hope this helps.