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
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.
Open the Settings app on the phone, then go into the Apps submenu.Here, tap the small icon next to the Your apps text, then enable the Show system apps option and press OK.
Now, scroll down until you find the Samsung Device Health Manager Service app in the list and tap it.Here, go into the Storage submenu, and hit the Clear data option at the bottom.
That should do the trick, and after a little bit of use or after you charge your phone, you should see the proper battery statistics again. Of course, if religiously checking battery stats isn’t your thing, you can simply wait for Samsung to fix the bug with a future update, whenever that comes out for your particular Galaxy device.
Source sammobile.. Hope the above solution works