Green line appears in Instagram

After exporting a vertical HD video using Final Cut Pro, and uploading it on Instagram to publish as a reel, in the second-to-last step of the reel upload prompt, Instagram displays a green line along the right edge, indicating how the video will look after it is uploaded.

Resolution of Instagram reels is supposedly 1080x1920 so I don't understand why a green line would be added, seeing as the exported video is the same resolution so should fit 1:1.

I tried exporting from FCP to different formats at the same resolution, including MOV vs MP4, H264 vs not, etc, as well as converting the FCP output to another format after importing into Adobe Premiere Pro. The solution that seemed to have a higher rate of success was using Adobe's Premiere free online MP4 converter. But even then, the online converter (1) didn't always work in preventing the green line at that step, (2) doesn't accept videos over 2 GB to process, (3) compresses slightly more than I'd like, which may be exacerbated by point #2, and (4) relies on an online service that could disappear at any time.

I haven't had the green line issue with uploading the same video on other services I use including TikTok and YouTube. I presume it's probably an Instagram issue, and I already reported it to them. However, if anyone knows of any [other/better] workarounds involving FCP, I'm willing to try. Thanks in advance.