Just upgraded to BIAB 2020 after installing Big Sur and I'm experiencing the same issues others have reported with the Notation Window.

macOS Big Sur Version 11.0.1 (20B29)
MacBook Air (Retina, 13-inch, 2019)
Processor 1.6 GHz Dual-Core Intel Core i5
Memory 8 GB 2133 MHz LPDDR3
Graphics Intel UHD Graphics 617 1536 MB