Most of the time, even if the phone has finished charging, the lightning bolt on the battery icon persists until the phone has been unplugged, so I can't tell if my battery is at 100% unless I pull the notification shade down or lock the phone. Once or twice, after leaving my phone charging for a while and then returning to it, I've noticed the lightning bolt has disappeared – which is surely expected behaviour – but I'm guessing this is a while after it's hit '100%', so maybe it's a calibration issue? Or have other people on the Oreo beta had the same problem?
I think all Nokia 5 beta users are experiencing this including me!! After the percentage reaches 100, the lockscreen shows 'charged' but the battery menu in settings shows it's 'charging' and the same goes in the status bar battery icon indicating a lightning bolt. This lightning bolt disappears after few minutes and shows 'charged' in settings as mentioned by you. I wait until the bolt disappears and then unplug the phone.
I have reported this in the Feedback app a couple of times. Hoping a fix for this in the stable build.
If I let the phone charge until the icon changes, I get about an hour's use before it drops below 100%, so the OS is clearly misreporting how much charge there is. Still remains to be seen whether this will be fixed with the stable release or the battery needs recalibrating.