hi will,

i think this is due to the DataRegion being loaded asynchronously. try something like:

var params = LABKEY.ActionURL.getParameters();
    var qwp1 = new LABKEY.QueryWebPart({
        schemaName: 'assay.example.' + LABKEY.page.assay.name,
        queryName: 'Data',
        filterArray: [LABKEY.Filter.create("Run/Rowid", params["Data.Run/RowId~eq"]), ],
        renderTo: 'ResultsPartDiv',
        scope: this,
        success: function(dr){
            if (dr) {
                console.log('Got data region');
            } else {
                console.log('Could not get data region - Bug?');
            }
        }
    });

and see:

https://www.labkey.org/download/clientapi_docs/javascript-api/symbols/LABKEY.QueryWebPart.html#constructor