iMessage read status not syncing

I'm running macOS High Sierra 10.13.6 on my MBP and iOS 11.4.1 on my iPhone 6s. Sometime in the past couple weeks or so, I've been noticing that the status of iMessages is not syncing between my devices properly. Specifically, I'll get all the messages on both devices but they'll not get marked as read on all devices like they used to all the time. The notification of getting iMessages on both devices seems to be a lot farther apart in time than they used to be. It seems to be a timing thing that happens more often when I'm in an active back and forth conversation on one device. If I'm away from both devices and I come back and they both have the same messages loaded and all unread, when I read them on one, they do seem to get successfully marked as read on the other.

I am _NOT_ using iMessage in the Cloud.

Anyone else seeing this?