Messages (SMS) Read Status Not Syncing Across Devices (solution)

For almost a year, when I read an SMS message on my iPhone, it would not show up as read on my other devices. Apple tech support was of no help (and wanted me to remove MDM and erase all my devices). I have also seen numerous Apple support threads (that are closed) with no solution.

Today I found a solution on Reddit and am posting it here (since I can’t respond to all the closed support threads). The below worked for me. Thank you to reddit!

SOLUTION:

1. Under iMessage, unselect your phone number as a place to send and receive iMessages. You should only have your Apple ID/email address selected.
2. Turn off iMessage for 5 - 10 seconds. Just long enough for us to be sure it registered everywhere.
3. Turn iMessage back on. At this point you should only have your email address as the sender/recipient of iMessages.
4. Wait until you get the pop-up notification that “your APPLE ID is now being used for iMessage on a new Apple Watch”. If you forgot to turn off your phone number or if the notification says your PHONE NUMBER is being used for iMessage on your Apple Watch then go back to step 1.
5. After you get the notification that your Apple Watch is using your Apple ID for iMessage, you can change the iMessage settings to allow your phone number to send/receive iMessages. You can even make it the default. Just don’t unselect the email address and only leave the phone number.