When pretty permalinks aren't enabled, Optimization Detective sends the URL metrics data to the /index.php?rest_route=%2Foptimization-detective%2Fv1%2Furl-metrics%3Astore&_wpnonce=d6ef56f704. This ends up causing the request to fail with:

{
    "code": "rest_invalid_param",
    "message": "Failed to validate URL metric: rest_route is not a valid property of Object.",
    "data": {
        "status": 400
    }
}

This is because rest_route is returned, unexpectedly, as one of the params when calling $request->get_params():

The quick fix would be to unset( $data['rest_route'] ), but it would be better if the params we obtain are exclusively from the JSON body.

This code is also being touched in #1373 where the issue is also occurring.

Granted, likely only a miniscule number of sites don't have pretty permalinks enabled, so this isn't a critical issue.