Getting empty string for custom property when using expand=metadata.properties

Confluence Development Confluence Cloud
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1

I am trying to retrieve custom properties that I set using AP.confluence.setContentProperty() earlier. I am using AP.request() to retrieve all the pages in my space and examine their properties (using expand=metadata.properties), like this:

function getSpacePages (spaceKey) {
    return new Promise((resolve, reject) => {
        AP.request({
            url: `/rest/api/space/${spaceKey}/content/page?expand=metadata.properties`,
            success: (responseText) => {
                let parsed = null;

                try {
                    parsed = JSON.parse(responseText);
                } catch (err) {
                    reject(err.message);
                }

                parsed.results ? resolve(parsed.results) : reject(parsed);
            },
            error: (xhr, statusText, errorThrown) => reject(statusText)
        });
    });
}

When I look at my results, I see the following:

Screen Shot 2017-10-20 at 11.32.26 AM.png2478×746 126 KB

You can see that my property (metadata.properties._expandable.validation) is an empty string. However, when I look at the page properties for that same page using Confluence Property Tool add-on, I see my property as it should appear:

{
  "key": "validation",
  "value": {
    "user": {
      "fullName": "Zena Hirsch",
      "id": "admin",
      "key": "8a7f80835f03104d015f075b0c530048"
    },
    "date": "2017-10-20T11:27:20-04:00",
    "validated": true
  },
  "version": {
    "number": 7
  }
}

Can anyone explain what I’m doing wrong? Thank you!