Apps that I've written to implement split-screen mode for iPhone Plus devices in landscape mode no longer implement split-screen mode when built and run on the iPhone Xs Max. For example the detail view cannot be constrained to right side of the screen while the Master view is constrained to the left side of the screen, but rather the detail view is extended over the entire screen. This unexpected behavior occurs only on the iPhone Xs Max and not on the earlier, iPhone Plus device models.
An example of the expected UI behavior is what you see in the built-in Apple Mail and Calendar apps when you rotate an iPhone Plus device to the landscape orientation.
Interestingly, the Apple Mail app has unexpected UI behavior on the iPhone Xs Max for iOS 12 (through 12.1, beta 2 so far); wherein the mail message headers on the left side of the screen partially obscure the body of the selected message that is normally entirely confined to the right side of the screen. This unexpected behavior in the Mail app does not occur on the earlier iPhone Plus models and is therefore probably a bug (reported).
One can conclude from the WWDC 2018 video entitled "Building Apps for iPhone XS, iPhone XS Max, and iPhone XR"1 that iOS 12 implemented a change to the behavior of the SplitViewController API associated with the .automatic value of it's preferredDisplayMode property.
If you specify preferredDisplayMode = .allVisible for your UISplitViewController instance [e.g., in the AppDelegate method application(_:didFinishLaunchingWithOptions:)], you should then get the same expected split-view UI behavior on the iPhone Xs Max in the landscape orientation as you would get on the earlier iPhone Plus models.