Like it does every year in December, Apple will shut down for a week to celebrate the holidays. This means apps cannot be uploaded or reviewed during this stretch of time (Dec. 23 - 27).
Between Dec. 23 and 27 Pacific time, new App Store submissions will not be accepted.
At the same time during December 2018 Our engineering teams are doing major maintenance to our iOS app build and publish process. The mechanisms that the legacy build process had to publish on the Apple Store were not compliant with Apple's guidelines it was unstable, full of workarounds and it didn't support 2FA. We are now taking a different approach to it and updating our publishing process with the latest most modern frameworks supported by Apple.
Considering the expected delays, We’ll do our best to have your app submitted before and after this window!
You can check this link for Apple official communication: