The documentation for Cesium and Webpack appears to be broken around 1.47, 1.48 . Whenever the iframe implementation was enforced as the dom container for the map.
Heads up, I figured it out.
CESIUM_BASE_URL : JSON.stringify('')
now needs to be
CESIUM_BASE_URL : JSON.stringify('../')
From https://github.com/AnalyticalGraphicsInc/cesium/issues/6319#issuecomment-410764699