Bound values not updating within amp-render

### Description

I'm trying to implement a version of the [Favorite Button example](https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/) using the newer `amp-render` component rather than `amp-list` with its `single-item` workaround.

Note that the button itself (`input type="submit"`) has a bound `class` attribute.

However, when using `amp-render`, these bound values do not update, despite successful XHR and `AMP.setState` actions taking place.

### Reproduction Steps

In my real-world code I've also changed the element to `button`, but the behaviour is the same whether it's `button` or `input`, and bound to `[class]`, `[text]`, `[value]`, or anything else: the binds simply do not update.

Simply change the `amp-list` to `amp-render` to duplicate:

```

```

### Relevant Logs

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### Browser(s) Affected

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### OS(s) Affected

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### Device(s) Affected

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### AMP Version Affected

_No response_

Start with the Favorite Button example and the amp-render reproduction, comparing it with the amp-list version. Trace the AMP.setState action and successful XHR through amp-render to determine why the [class], [text], and related bindings do not update; done means the reproduced bindings update after the state change.

Written by the indexing model from the issue text.