I guess that either the widget defines something that already is used within mxui, or (the old instance of) the widget is still there while a new page is loaded. If the second scenario is the case, I guess you would need to look at widget data clearing / re-initializing.
So I figured this out in the meantime:
So somehow I broke widget loading for all widgets after my widget gets loaded.
Already tried to minimize dependencies but my widget does depend on https://gojs.net/latest/intro/react.html. That might be causing the issue, but still do not see how.