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.

: int will be deleted from the function.

The same issue occurs in Qt Design Studio 4.