The iOS 12.3.1 update is a small maintenance release and it could have a noticeable impact on your iPhone 7’s performance.

The iOS 12.3.1 update is performing well on our iPhone 7 models right now, but some users are running into trouble after making the move.

iPhone 7 users are complaining about a variety of iOS 12.3.1 problemsincluding weird battery drain, Bluetooth issues, Wi-Fi problems, and issues with first and third-party applications.
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https://www.gottabemobile.com/7-things-to-know-about-the-iphone-7-ios-12-update/