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if it's iphone footage then it was probably recorded in variable frame rate. you should transcode your footage to constant frame rate first using Handbrake. try https://forums.adobe.com/people/R%20Neil%20Haugen workflow here: Re: Iphone clips are in slow motion when imported
How did you get the movie from the phone to the computer? Sometimes the file gets truncated during the transfer. If it's an iPhone, connect the phone to your computer and open the Preview app. Under file you'll see something about connecting to your iPhone. Click on that and download the files directly that way. Then you should be able to drop the file right into After Effects without problem. I've done it that way dozens of times without any special CODEC. If that doesn't work, try opening the file in Adobe Prelude and converting it to an H264 or ProRes.
Hi Daniel E. Sage,
If this is iPhone 8 HEVC footage, then you will have to transcode it. Transcoding in general fixes this issue. Try it with Handbrake.