For a while now I use an IPhone as well for the videos I create (shameless self-promotion:
https://www.youtube.com/c/PartituraOrganum). In the beginning I had the same problem, the video was slightly slower than the sound recording. As I use Vegas as a video editor, that allows you to edit sound independent from video, I could slow down the music a bit and make them match again.
The problem was that the videosettings of the IPhone and my project settings in Vegas were slightly off. The frame rate of the IPhone is 29.97 frames per second and my project settings were 30 frames per second. Or the other way around, I forgot the exact details. Matching the project settings' frame rate to the IPhone's solved the problem for me.
It's a bit odd you experience this problem with IMovie. I would expect Apple to make sure IPhone and IMovie work along correct right out of the box.