@RDTuin: Can you try heading to Settings > Apps > Choose Default Apps > Opening Links > Open Links In Apps > On > Scroll down and select Facebook > Open Supported Links > Always Ask. Let me know how you get on.
EDIT: Looks like I posted this in the wrong community. I meant to post this in the Australia community page as it might be a regional issue. Ignore if you don't see similar issue in the EU version of One UI 4.0
The update finally appeared on S21 Ultra and I updated last night. I too use dark mode all the time - light sensitivity issue.
One thing that I find annoying with this new update is that when I launch the weather app, it comes up in a bright light-blue background totally destroying my dark mode experience. However, if I scroll up slight on this bright screen, I get dark mode experience back - why oh why???