[JS] Adding breadcrumb `data` results in either default breadcrumb with no data or event not being sent entirely

I am trying to use beforeBreadcrumb to pass along more error information from a network response, and I find that if I modify the breadcrumb before returning it, for example with breadcrumb.data.errorMessage = response?.message;, the breadcrumb shows up in the log as a generic/default breadcrumb with no data associated. Logging the breadcrumb directly before returning it shows the expected data is still there:

    "timestamp": 1642071435.622,
    "category": "fetch",
    "data": {
        "method": "POST",
        "url": [redacted],
        "status_code": 401,
        "errorMessage": "The user credentials were incorrect."
    "type": "http"

But none of this data is shown by the Sentry UI.

To troubleshoot, since the processing in the implementation is involved, I tried some simple debugging with this form of the hook:

    beforeBreadcrumb(breadcrumb, hint) {
      breadcrumb.data.errorMessage = 'xxx';
      return breadcrumb;

This results in the error not being surfaced in Sentry entirely. When commenting the data override line, the error is processed as normal.

Here I’m at a loss, as the docs indicate to add custom data under breadcrumb.data. Any ideas? Many thanks in advance!