String Objects arg() method not working properly inside a QML function

Within a QML function, I want to use its argument as the argument of a string objects arg() function. But this does not work as expected. It only works, if I store the parameters value into a local variable first and then use this local variable as the argument for the arg() function. To illustrate that, I created a simple example project:

As you can see, without the intermediate step of storing the functions argument into a local variable, the arg() function will not replace the "%1" with the given argument.

Expected/Desired Behaviour:
When using the QML functions argument directly for the arg() function, the "%1" in the text will be replaced by the given argument.