https://www.twitch.tv/.../930239140 | Why are my screens so pixelated in some of my Twitch streams? And how can I fix it? By the way, I use OBS Studio.
These are compression artifacts, you have too low a bit rate for 1080p50
Sounds like too low a bit rate. Up to 6,000 Kbit / sec are possible with Twitch.
Look what you have set and whether your internet connection allows you more.