I've also noticed this issue - with the notification slider set to vibrate, my phone will sometimes ring at full volume (although not always)
I have also had an issue with this. Haven't noticed it because of it being unattended, as I constantly check my phone, but when I'm sitting at work...
Forgot to include that the app doesn't have an app-level setting that can be adjusted.
In Settings > System > Date & time, turn off "Use 24-hour format".
Open the OnePlus weather app.