Frequently we see trouble on this forum with iPhone video. We assume it has something to do with variable frame rates. Recently I was given eight iPhone clips from an iPhone 8 to put into a video. MediaInfo says it is "Frame Rate Mode: Variable" at 30 fps.
I had absolutely no trouble using the clips in a 1920x1080p30 project and even added some 4K footage from a different camera as well as several JPEGs.
It all worked without a hiccup using PrE 2018 on Windows 10. Flawlessly. (I do have Quicktime installed on my computer.)
I think that when users say they are having trouble with iPhone video there must be some other answer than "variable frame rate".