As the sun sets, the shadows goes up the mountain face. Simple geometry says that the shadow can't go up the mountain face unless the sun is sinking behind something. The "cloud lit from underside" theory doesn't accomplish anything here. We aren't talking about light sneaking in somewhere unexpected; we are talking about a shadow, the absence of light.
What's creating the shadow? What is blocking the sun's light that so regularly crawls up the mountainside, finally leaving it in shadow? If it is already dusk, why can I fly up in a plane and see the sun again? I haven't found an explanation in Flat Earth theory that explains this. Not saying there isn't one: if there is, I'd like to hear it.
I think that all of your questions would be adequately answered if you just read the wiki page on EA: https://wiki.tfes.org/Electromagnetic_Acceleration.
Yep, it does appear that this EA theory provides an explanation for the phenomenon. Not saying I accept it; just saying that if one does accept it, it explains the shadow.
Just for the record, I wasn't straw-manning (great word). I did say an "apparently" endless plain, which means nothing can be SEEN breaking its uniformity. Not proposing an actually endless plain, would indeed be straw-manning.
I've enjoyed this discussion. Thanks, all.