Be aware that iOS 12 is a beta software and bugs are to be expected. This is probably a beta software problem because I know other people that have this problem too. It was fixed by downgrading their OS to iOS 11.
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[https://www.macworld.co.uk/how-to/iosapps/downgrade-ios-12-beta-3522302/|How to Downgrade to iOS 11]
Be aware that iOS 12 is a beta software and bugs are to be expected. This is probably a beta software problem because I know other people that have this problem too. It was fixed by downgrading their OS to iOS 11.