Xcode SDK Version Issue

I'm having a bit of an issue with distributing an app via the latest version of Xcode (Version 10.1 (10B61)). Every time I try to upload it to the App Store via organizer, I get this error after uploading:

WARNING ITMS-90725: "SDK Version Issue. This app was built with the iOS 12.0 SDK. Starting March 2019, all iOS apps submitted to the App Store must be built with the iOS 12.1 SDK or later, included in Xcode 10.1 or later."

Previous answers to this question said that it was just a warning and that I should still be able to upload my app, but it is now March and I am still getting the error even on 10.1. iTunesConnect will not let me select these builds and I get emails about the validation failure.

Further details:

-The app is an iMessage extension, and uses exactly 1 framework from Carthage, which is up to date with the latest framework version and Carthage version.
-My project and Message extension targets Swift Language Version build setting is Swift 4.2.
-My deployment target is iOS 11.0, but the issue persists after selecting iOS 12.1.
-The app compiles and runs fine on my physical device running iOS 12.1.
-The issue persists after a build folder clean.
-The issue persists after reinstalling Xcode.

If anyone has any suggestions as to how I can verify my app is compiling with the latest SDK please let me know.

Whether the app runs on given iOS version, has no relevance to which SDK are you really using. The definitive SDK version used for building is found in the app's Info.plist.

1. after Archiving, Show the xcarchive in Finder.
2. right-click, Show Package Contents.
3. open Products/Applications/YourApp.app
4. right-click, Show Package Contents.
5. copy the Info.plist to some place where you can run terminal commands on it
6. /usr/libexec/PlistBuddy -c "print :DTSDKBuild" Info.plist

You would expect it to show "16B91" for SDK 12.1. If it's something else (SDK 12.0 would be "16A366"), here are some ideas:

• inspect the Carthage dependency. Are you really building it locally or downloading a prebuilt binary? If in doubt, force local building with --no-use-binaries. Remove whole Carthage folder (Checkouts as well as Build) and start from scratch.
• rm -rf ~/Library/Developer/Xcode/DerivedData instead of just "build folder clean"
• Verify MacOS System Report section Software/Developer for the actual SDK versions that MacOS thinks that you have
• xcode-select -p to verify that you are really using the Xcode instance that you would like to

If all looks correct on your side, fill Technical Support Incident. Report the exact rejected build which you uploaded already. That should speed up the support reaction.