Thanks for all that great data.
You are correct that not all Mp4 files work in iMovie. It must be Mp4/AAC. AAC stands for Advanced Audio Coding.
If you have time and can do a little more experimenting per below, and post back with your findings, it may help us to track down the problem.
Your original clip has the letters HE in the codec. That might be short for HEVC, High Efficiency Video Codec. The exported clip does not contain the letters HE. Perhaps HE, if it means HEVC, is what is causing the problem. Put your original clip into QuickTimePlayer and do a Window/Show Movie Inspector. Does it show the format as HEVC? If it were recorded in HEVC, you can export it out from your iPhone and eliminate the HEVC by going to Settings>Photos>Transfer to Mac or PC and disabling "Keep Originals". Then export the clip. If you try that, see if the exported clip will work in iMovie.
Another thing you can do is try converting the troublesome clip to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures issues.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie. Then see if the converted clip exports normally in iMovie.
Another experiment that you might try is to import your original clip into iMovie and do a Modify/Detach Audio. Then delete the audio and export out only the video. If the video-only exports quickly that would indicate that the problem is with the audio codec.
-- Rich