I'm facing the exact same problem, the Device care app doesn't show any information about the battery usage for today and the past 7 days. Very weird
and I'm using an S20+ 5G
It is definitely a bug with OneUI 3.0, everything seems smooth when it is used but the things that needed more attention to detail were left out like battery life and optimization of apps.
Please feel free to call the call center and request remote assitance to help resolve the issue.