Thanks for using the Apple Support Communities.
I understand that you have to close the Messages app after reading a text message so you can read the next text. This started happening after you updated to iOS 11.
First, are you referring to the Messages app on the iPhone or a third-party messages app that you installed? If this is a third-party app, please see the steps in this support article: If an app you installed unexpectedly quits, stops responding, or won’t open .
If this is an issue with the iPhone Messages app, I recommend a reinstall of iOS. First, use this article to make sure that you have a backup of your iPhone: How to back up your iPhone, iPad, and iPod touch. Then, use this article to reinstall iOS: If you can‘t update or restore your iPhone, iPad, or iPod touch. Note: This article is intended to be for a different issue but following steps 1-5 will help you reinstall iOS without erasing data.
Let me know if you have any questions.All the best.