Same issue here. Just recently got a RTX 4070 gpu in order to be able to stream AV1. I have a stable internet connection with low latency, 30 mbps upload speed. Been streaming for years with H.264 with no similar issues.
Bitrate set to CBR, 2 keyframes, max 2 b-frames, p6, high quality, two pass(quarter res),look ahead off, psycho on.
OBS ver 30. Windows 10.
I stream sports to YouTube. Broadcast last for aprox 2 hours. First game I streamed it was 1080p50fps with 12mbps bitrate. Bitrate dropped to 160kbps after 20 mins or so. No obvious issues with the internet connection. Nothing in OBS logs. Tried a speed test from different devices/pc. Tried different things, but nothing I could do to fix the issue. According to YouTube studio the bitrate sent to YT is too low, indicating that the problem is not on YT’s end. I stopped and started the stream and it’s all good.
This Sunday I streamed a new match. Tried 1440p50fps with 18mbps bitrate. Worked like a charm for almost 100 minutes. Same issue as previous broadcast. Had to stop and start stream to get it going again.
Have a friend streaming same sports, but in a different town. He has not had any issues. More or less same settings as I. He uses an other ISP.
I will consider streaming via my 5G mobile router next game to see if this is more stable. I think I will get even higher upload speed and low latency. If this works I will switch back to the cabled ISP next game to see if issue is back.