I've not uploaded anything to Instagram, so not too knowledgeable about it.
Your vertical video exported out of iMovie will have black bars on each side. I don't know if this would solve your issue, but you might try exporting the video without the black bars, or removing the black bars, and then uploading the resulting video to Instagram.
There are three ways to export without black bars a 9:16 vertical clip from iMovie for Mac.
- You can rotate the 9:16 clip sideways with the rotation controls in the Crop feature. Export it sideways to your desktop. Open it in QuickTimePlayer. Rotate it right side up (Edit/Rotate). There will be no black bars.
2.You can go to the projects browser screen (where your projects are displayed as icons) and do a File/New App Preview. That will open a screen that looks like the normal project screen, except that it will have fewer features. Import a vertical 9:16 clip into the timeline. When you export the New App Preview (an option in the file export box) the vertical clip will export without the black bars.
3.Another way is to export a normal project with a vertical clip that has the black bars on each size Then play the video on the desktop. Use the Command-Shift-5 feature to drag select around the video to create a screen recording that eliminates the black bars.
-- Rich