This is because the widget is a custom creation, and not embedded as part of Mendix's platform.

I only foresee this being updated if that widget becomes a regular part of the Modeler. However, maybe someone from Mendix would know how to intercept validation calls sent to the client.

You could set up something like Fiddler to see HTTP requests. I'm honestly not entirely sure how the validation works.

In short, no this is not a bug, as it is a custom widget. Each widget made would have to be intelligently designed to handle all of Mendix's quirks and capabilities.