Same thing on my end! Thought I could reset the SMC but turns out you cannot really on m1. Kind of ridiculous that apple rolled out update without even fixing this issue in the betas...

Solution: It is based off the adapter. I had a doohalo adapter and I switched adapters to another brand (non apple) and it worked. My adapter doohalo was worked pre update on big sur. Sucks how apple is trying to find ways to get rid of certain brands that people are purchasing….