How to get updated function value into my view

Hi everyone,
I’m a newbie with Aurelia Framework. Thanks in advance for your help.

This is my problem:
I have an array FILTERS in my view model.

I would call the value of his first object in my view and I tried two ways:
1-Call it directly with string interpolation:

2- Call it through a function:

In the second case the value is loaded only once at the beginning and then is not updated anymore if I change it manually from an Input linked to it, for example.
In the first case it worked properly.

Why did this happened?