Your Phone app should mark the SMS as Read in our phone after opening it in Your Phone app. but currently it doesn't. when i open received text messages in my Your Phone app on Windows 10 and then after that go to my Android 9 phone and open the messages app, the received SMSs are Not read and i have to manually open and close them one by one to mark them as read.
@HotCakeX Would love this feature. I use Your Phone very often for SMS during the day; by the end of the day, I've got a huge backlog of "unread" messages on my phone that I have to manually clear out. Would be real nice to have that sync automatically. (If apps even support that API. Don't know.)