Hello @Will_Tsui!
I just replicated the component with a mapped value and also saw that my 'appTargetsByIndex' is also empty strings in an array. Very odd.
Will report this to our eng team to find out if I am trying to use it incorrectly. I was hoping to find an on click event handler but there doesn't seem to be one built in.
However, after searching breadcrumbs in the docs I found a potential alternative solution to your use case, check it out here as it involves the Stepped Container Component to navigate forward and backward through an app on clicks. Let me know if that works!