I have also had the dark mp4 render problem, and have tested quite a bit, I don’t know if this will apply to many of you but I have noticed I get dark mp4 renders with Bandicam recorded video but have noticed that I don’t get the dark mp4s if I recorded with Bandicam’s Motion Jpeg video codec instead of its MPEG-1 video codec.
I hope this can help some of you as it helped me.