Even though the two updates have a different number, they actually carry the same changelog, except that the OnePlus 6T gets optimizations for the display which the OnePlus 6 doesn't.
OxygenOS 9.0.4 and 9.0.12 updates bring in deeper integration with Google Duo to OnePlus 6 and 6T. What that means is you can now make video calls directly from the Phone and Messages app via Google Duo.
These updates also bring in general bug fixes and system improvements to both smartphones. And in addition to that, they also bump up the Android security patch level on OnePlus 6 and 6T to January 1, 2019. Which is a bit disappointing given the fact that Google already released the Android security patch for February.
OnePlus is rolling out these updates over-the-air to a small percentage of users and will commence broader roll out in a "few days". If you don't receive the update notification in the next couple of days, you can check for the update manually by heading over to the Settings > System > System updates menu.
It's not a lag by any means, still, when you see entry-level makers nowadays send the update much faster it raises the questions.
"OnePlus 6T gets optimizations for the display which the OnePlus 6 doesn't," that's great for 6T owners like me but what exactly does it mean?