Apple Watch - Unread Message Count

Using a Messages complication on my Apple Watch, the Unread count often shows higher than that on my iPhone.

I usually read messages on my iPhone, but they often show as Unread on the Watch.

To fix this, I have to go in to manually read all messages on the Watch. This can be a pain not only because I have to read the same message twice, but also if I haven't fixed the issue in a while, scrolling down to find the message can be a bear.

I already have Message Notifications set to Mirror My iPhone, as was suggested in this thread: Text messages - Apple Community

All devices are constantly up-to-date, and regularly restarted.

Any ideas how to fix this?

Thanks.