Same problem here.
Specs:
- Macbook Pro16 with Core i9
- 32 GB DDR4
- 1TB SSD
- Radeon Pro 5500M 4 GB
Automatic GPU switch (on/off) - It does not matter, the problem still exists.
Connected to the LG 4K monitor directly via TB/USB-C.
Each time after sleep or restart my Mac, the resolution of the monitor changes.
The availability of the option to select the resolution also changes, 1080p, 720p sometimes 1024x768 is MAX what i can get.
Other Mac with BigSur work's well (Macbook Pro 15 / 2017 / 16GB DDR3/ 256 SSD), i can get 4k 60Hz (no problem).
So APPLE must broke something with our Macbook Pro 16 with BigSur update i think..
One note, the connection via the HDMI adapter (original or for example satechi) allows me to 4k at 60Hz without any problems.. So it must be something with usb-c direct connection ..
I try to reinstall OS by wipe out partition, and install fresh BigSur, but problem still exist.
I try to reset NVRAM/SMC - no luck, problem still exist.