These are visual representations for both models, neither of which bend light to go under the cloud because quite frankly its impossible.
On the round earth model the light can clearly hit the underside of a cloud if enough room is made. You also have to remember that you dont get red sun sets every day, simply because the conditions need to be right. i.e clear sky between the suns rays and the clouds its hitting.
The round earth model works differently. The sun can not directly hit the underside of the cloud. The suns light is refracted at extreme angles and as it hits the top of the cloud the light is scattered through it creating the pinks and oranges we see at sunrise and sun set.
These images are exaggerated for demonstration purposes only.