Preview link off PR into the main branch; an update to main branch does not trigger a rebuild of the preview link

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- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-hosting/blob/master/CONTRIBUTING.md).
- [X] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.
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### App Id

d2kru5yqfil20b

### AWS Region

us-east-1

### Amplify Hosting feature

Web previews

### Describe the bug

Preview links are not updating when the parent branch of the PR is updated with a new commit

### Expected behavior

if I have a PR from `feature-1-branch` to `main`, I want to see this preview link updated

1. when a new commit is added to the PR branch (feature-1-branch)
2. when a new commit is added to the "parent branch" of the PR (main)

Right now, I don't think 2 is happening

### Reproduction steps

You should see this with any preview link

### Build Settings

_No response_

### Log output

```
# Put your logs below this line

```

### Additional information

_No response_

Start with the Web previews feature and trace how commits to a pull request branch and its main parent branch trigger builds. The report names no source files, tests, logs, or build settings, so first reproduce the behavior with a preview link and locate the relevant event entry point. Done means a new commit on either branch rebuilds the preview link.

Written by the indexing model from the issue text.