The strange flaring almost certainly comes from a dirty sensor. If you look closely on your stopped down videos you can see a dark smear. When this smear crosses the light point it flares. The smear exhibits on both lenses, hence it must be on the sensor.
Good review and information! I've owned the Samyang 45mm f1.8 for years, it's my only modern lens and I'm very pleased with it. Why do you limit your audience to the only country left which uses antiquated measurements? Saying or putting up text for us folks in the rest of the world would make your videos more welcoming to us, at a minimum. There is a huge world outside the USA.
