Apple has released an updated build of iOS 12.1.2 for iPhone. The updated build is effectively the second release of the latest maintenance build for iOS that was released on Monday. The new build is 16C104.
Why a second release of the update has happened is not clear. There are no changes in functionality or fixes in this new build. It is still aimed at fixing issues with eSIM activation in the iPhone XS lineup and the iPhone XR. It also addresses a cellular connectivity issues in Turkey. Perhaps the biggest change is that it addresses the Qualcomm patent issues in China. In all likelihood, given that Apple rushed to get iOS 12.1.2 out in response to the China situation, there was a bug that was found after the fact and this second release fixes it.
What is for sure is that if you have already upgraded your iPhone to 12.1.2 you will not see this update. It is only being made available to those iPhone users who had not upgraded yet. If you have not upgraded yet, you can go to Settings>General>Software Update on your iPhone to download it. If you have upgraded already, you will not see the update yet.