That's the issue here: whether the sun is always above everything on a flat earth, or whether the earth is a globe making it possible for a brief amount of time for the rays of the sun to be coming from a source closer to the ground than a tall object relative to that object's flat and level position
I am not suggesting that the sun can cast a shadow from behind the curve of the earth either. Right when the sun is visible on the horizon, and for a very short amount of time after, the sun will be shining from a lower point than the top of tall objects relative to their position. Any lower, and those objects are in the earth's shadow. Any higher, and the sun will be shining from a relative higher position. To experiment with this and try to film it actually happening would not be easy. It should not be done on land unless the land is bare and has very minimal elevation changes for several miles, so any hills and trees don't block the sun until it is already relatively above the top of the tall objects being used to cast shadows. Fog, or a backdrop of some sort will be needed to see that the shadows are being cast up, and not horizontal or down. A plane or helicopter should also work to put the viewer in the shadow, but still able to see over the object casting the shadow.
One issue I just noticed about my image here is a problem of perspective: the sun is actually much bigger in the globe earth model even though it looks small to us. But, even with a poor representation of a larger sun (see below), far from accurate scale, there will still be a shadow behind the earth, and 2 parts of the earth where the sun is relatively level with the ground, or slightly below tall objects.