I have upgraded my Apple Watch Series 2 38mm to watchOS 4 recently. The SMS app shows all messages remain unread with the blue dots by the message threads, while the iMessages app runs normally. However, I had already read some of them on my paired iPhone X with iOS 11. I takes a while to read all in my Mac and the SMS are still left unread on the wearable device. It seems that the Messages doesn’t run in the background to mark them as read. That never happened in watchOS 3 version. How can I get it working again?
How to fix SMS messages remain unread on Watch after read on iPhone X
Make a restarting with all devices
* First Power down both devices
Press-and-hold the side button of your Apple watch until the Power Off slider appears in the screen. Then drag the slider to turn it off completely.
On your iPhone X, open up the Settings app, tap General, scroll down to the bottom and tap on “Shut Down”, and then move the “slide to power off” option to off.
* Then bring up the iPhone X. Simply press and hold the Side button of the device for a few seconds to get it back on.
* The next step is to bring up the Watch. Press and hold the side button to get the Apple logo and release them. Once you’ve done that, all your messages will start notifying of you on the small screen. After the syncing process is finished, the related messages will mark as read.
If the above method is not working, the final method is to erase all content and settings on your Apple watch and then restore it from a back up.
* Open the Watch app on your iPhone, go to My Watch tab, tap General > Reset > Erase Apple Watch Content and Settings, and confirm that. Then tap Start Pairing > Restore from Backup. After everything stays in sync, the problem will be resolved.