I came here with the same issue.
Only when using OBS does my stream start fine and with 30-40 seconds it goes to 0 kbps. If I upload a file to my server it'll stay at 10-12 mbps all day. OBS kills it with 100% frequency. I've been using OBS for nearly a decade, never seen this before.
My ISP has told me they can do nothing as it's not on their end.
Ethernet not wifi. New cables. All drivers and updates are in place. Firewalls, defender turned on/off for testing to no affect. All network options within OBS tried to no affect. Uninstall/resinatll with reboots at every step tried. I currently can not use OBS to stream. Even tried eliminating it being a camera issue, display driver, network driver, audio driver, power saving issue, anything and everything.
Stumped and annoyed.
Anyone with anything left to try (or an answer?)