I finally figured out what is causing it, and am able to reproduce it in a minimal fiddle.
You were right that it was not angular or webpack at all, it was actually Bootstrap 4 beta CSS which is causing the issue, which I guess makes more sense!
Please see this fiddle where the error occurs. All I did was take your basic pie chart example, and add bootstrap 4 css to external dependencies on the left side of the fiddle, in their menu.