It’s not possible for sunlight to be cast upward from underneath clouds on flat earth. Unless maybe FE think that the sun is lower than clouds. This would be notable since people would notice the sun immediately above the ground.

Sources:

• https://www.amusingplanet.com/2013/10/the-shadow-of-mount-rainier.html
• https://www.reddit.com/r/mildlyinteresting/comments/5ba92u/mt_hood_casting_a_shadow_on_the_clouds_this/
• https://flatearth.ws/shadow-on-clouds