So I can’t send the video (don’t have permission from person filmed), and decided to just create a really short test video and send it to you. It did not have a problem! After trying a few things, it appears to be correlated with video length. Cubase does not support the format of any video I create longer than approx 2s, but can import any video shorter than that.
iOS 12.0.1 (16A404)
iPhone Model A1522