Streamers on cable internet often notice the same pattern. Morning streams are perfect. Afternoon streams are fine. Then around 7 or 8 pm — right when the audience is biggest — dropped frames start piling up and the stream turns to a slideshow.
The reason is called peak-hour congestion. Cable internet is a shared medium — you and every neighbor on your local node share the same upstream pipe. During evening peak hours, everyone else is also streaming, gaming, and video calling. Your advertised 20 Mbps upload can quietly drop to 10 or 12 Mbps sustained. If your OBS bitrate assumed 20 Mbps was always available, your stream falls apart every night at the worst possible time.
The two fixes are dropping your bitrate to survive congestion, or moving to fiber, which is not shared the same way and gives you consistent upload all day. If you want a clean breakdown of Twitch's actual technical requirements and how to test your real upload during peak hours, this SpeedTestHQ guide to Twitch streaming upload speed requirements is a solid reference.
