The more I think about it, the more it seems to me that I made a mistake by adding "move away from the 'walls'" portion of the script (in the "for each item" block) to the custom block.
I am beginning to see that it is doing the opposite of what was my intention.
My intention for the clones was to point towards the closest pixel of the 'walls' (but this is not what the "point to" block does) and then move in the opposite direction.
I forgot that the "point to" block, instead, points towards the center of the sprite, which I don't want, at least not in the case of 'walls' sprite (because it's center is in fact outside of the costume's pixels).