Quality downscale after uploading to Instagram

Although some disagree with my approach, I would refrain from using two pass encoding for social media.

The logic is that social media uses a pass-the-hot-potatoe approach due to sheer volume of things being uploaded. 2-pass encoding means that the entire file is being optimized for file size, not necessarily quality. It is therefore great for use on local playback systems, rather than social media re-encoding.

See if you get better results with 1-pass VBR encoding or even constant bitrate at higher levels. The less processing has to happen at the social media end, the better the results will be when they're done.

Hope this helps.

You may have seen this, but it looks interesting:

https://www.adobe.com/creativecloud/video/hub/guides/upload-high-quality-instagram-videos.html

I would try exporting from PP in ProRes (LT), and upload that, it often works well in YT.

If your export is high-quality coming out of Premiere, the issue is likely on Instagram's side.

It's important to note that most platforms will re-encode your video to their own format and settings. When this happens, especially with high-quality video, the initial streamed version may be standard quality. It can take another 10-20 minutes of processing (depending on size and length) before the high-quality version becomes available.

Cheers,
Paul

There's an option in the  Instagram app to "Upload at highest quality".  Is that enabled?