Solution 1: Force Close Message AppIf you open Messages app only to find the messages not in right order, then the first solution you can try is to force close this app by tapping home button twice continuously and then slid up this app. After that, Reopen this app to see if it helps.
Solution 2: Disable iMessage And Then Enable It AgainSometimes, the problem may be caused by iMessage. You can go to Settings > Messages > iMessage to disable it and then restart your device to enable this setting. This solution really helped many people!
Solution 3: Reboot Date&Time SettingNormally, the default setting of date and time is Synchronized with network time. And it is said that a reboot of this setting can solve this messages out of order problem. Therefore, you can enter Settings > General > Date&Time > Set Automatically to disable it. After exiting this setting for 15 seconds or so, re-enter to set it to Set Automatically again to see if it helps.
Solution 4: Force Restart Your iPhone
If this problem is caused by app crash or something like this, then this solution can be a good way to solve this issue. If you don’t know how to restart your device, follow the guidance below:
For iPhone X, 8, and 8 Plus: Quickly press and release the Volume Up button and Volume Down button respectively, then hold the Side button down for 10-15 seconds until you see the Apple logo.
For iPhone 7 and 7 Plus: Press and hold the Side button and Volume Down button together for about 10 seconds, until you see the Apple logo appear.For earlier iPhone models: Press and hold the Side button and Home button together for about 10 seconds until you see the Apple logo appear.
Solution 5: Reset Your Device to Factory SettingsNo Matter what causes this problem, reset your device to factory settings will remove this bug unless there is hardware problem inside your device. Go to Settings > General > Reset > Erase All Content And Settings to reset your iPhone. After that, you can use Jihosoft iPhone Data Recovery - Recover Data from iPhone, iPad & iPod Touch to get your data back.
Solution 6: Update Your Device to Newest iOS VersionIf none of the solutions above can solve this issue, you can try to upgrade your iOS device to the newest iOS version. This method is not listed as top solution because every new iOS version has some other bugs or problems since iOS 11 according to iPhone users. Therefore, you can try other solutions ahead of this in case of other side effects.