Tried to replicate your issues below but it worked fine for me. I didn't try with more than 11 records but I could adjust the duration to be whatever without anything breaking. Are you returning the end_date value from your data resource? 

Roadmap items:

/**
 * @Param {params} params
 * @Param {api} params.api
 * @Param {TransformApiHelpers} params.helpers
 */
function evaluateProperty({
    api,
    helpers
}) {
    const items = api.data.look_up_multiple_records_1.results;
    const ret = []
    items.forEach((item, i) => {
        ret.push({
            id: {
                value: i,
                displayValue: i
            },
            planned_start_date: {
                value: item.work_start.value,
                displayValue: item.work_start.value
            },
            planned_end_date: {
                value: item.work_end.value,
                displayValue: item.work_end.value
            },
            short_description: {
                value: item.number.value,
                displayValue: item.number.value
            }
        })
    })
    return ret;
}

Metadata:

/**
 * @Param {params} params
 * @Param {api} params.api
 * @Param {TransformApiHelpers} params.helpers
 */
function evaluateProperty({
    api,
    helpers
}) {
    const ret = {
        "startDateField": {
            "name": "planned_start_date",
            "label": "planned_start_date"
        },
        "endDateField": {
            "name": "planned_end_date",
            "label": "planned_end_date"
        },
        "itemNameField": {
            "name": "short_description",
            "label": "short_description"
        },
        "itemOwnerField": {
            "name": "assigned_to",
            "label": "assigned_to"
        },
        "groupableFields": [],
        "colorableFields": [],
        "metricsFields": []
    }
    return ret;
}