Type annotations are deleted when QML files are reformatted using Tools menu → QML/JS → Reformat file.

Steps for reproduction:

To any QML file, add an arbitrary function whose arguments have type annotations.

For example:

function foo(i: int) {
    console.log( i )
}  

Reformat the file using the above command (show the Tools menu first if it is hidden in QDS 4.x) 

: int will be deleted from the function.

The same issue occurs in Qt Creator 10.