As part of making Phonegap apps work full screen on iPhone X and XS, you need the splash screen plugin with specific splash screens configured to make the app load in true full screen.
As part of this, you need the viewport meta tag to include this: viewport-fit=cover
This then stops the KeyboardShrinksView setting in config.xml (and the JS function too) from working.
Removing this: viewport-fit=cover
from the HTML meta tag fixes the issue, however then you don't get the full screen on iPhone X and XS, and instead you get white bars on the top and bottom.