AMP Form - input [value] won't allow concat() function

Question

In AMP Form, I am trying to fill in a hidden input field based on a form response. The response is an array inside of a JSON object. When I try to pull in the information into a div's [text] attribute, it works. But in the input's [value] attribute, it won't.

Here's my sample code:

<form method="post" action-xhr="submit.php" target="_top"
    on="submit-success: AMP.setState({answer:{display:event.response.answer}})">

    <div id="response" [text]="answer.display.concat()"></div>

    <input type="text" name="answer" value="" required>
    <input type="hidden" name="active" value="" [value]="answer.display.concat()" />

    <input type="submit" value="Submit" />

</form>

After I submit, the div with id "response" will display the event.response.answer information but the hidden input's value will not change.

However, if I change the answer.display variable into a string instead of an array, the input's value will work. For example, this works as expected:

<form method="post" action-xhr="submit.php" target="_top"
    on="submit-success: AMP.setState({answer:{display:'foobar'}})">

    <div id="response" [text]="answer.display"></div>

    <input type="text" name="answer" value="" required>
    <input type="hidden" name="active" value="" [value]="answer.display" />

    <input type="submit" value="Submit" />

</form>

So why doesn't concat() work inside of an input's value attribute even though it will work with a div's text attribute?

And maybe more importantly, how can I use the form's response in my form so that it can be submitted again?