It appears that some streamers have access to an enhanced 1440p stream limit with HEVC (original blog post). Streamlink doesn't yet have support for the "experimental" codecs such as HEVC (h265) and AV1. Additionally Twitch using HEVC is an awful decision as Firefox still doesn't support HEVC. Supposedly it's in the nightly builds so it might be a awhile before it's properly released, knowing Firefox. AV1 was supposed to solve this issue 🤷‍♂️.

I'll keep this issue updated with findings or if anyone has any insights post them here.