The sides of the top of the scaffolding blocks above the air is not solid

The scaffolding blocks above air are:
1.- Solid at the top when the player tries to go down (he can not).
2.- Not solid above and below when the player tries to climb (he can).
3.- ALWAYS not solid on the sides of top.

"1" and "2" are good functions... but "3" gives problems to use.

The "3" causes that:
When the player walks to the side (half-body,), the sides of the top of the scaffolding blocks above the air is not solid, the player fall!.

What happens (image1):
When the player moves to the left, he will fall, because he will not collide or stop with the top of the scaffolding block above the air because it is not solid.

I expected (image2):
When the player moves to the left, he will not fall, because he will collide and stop with the top of the scaffolding block above the air because it is solid.

Final comment (image3):
If this were to change, and the sides of the top of the scaffolding blocks above the air were solid, the 'solidity' should be a very thin layer, only the top... so the player can go under without hitting (as maximum, similar trapdoors).