Unfortunately we could not solve this issue, due to the lack of documentation.
Basicallly we don't know if we're using the right procedure to register the toolbar, because there is not any sample or reference in the SDK we can use. Our request to Autodesk about this issue, never was solved.
But in Max 2018, they changed how toolbars are handled, and now are driven by the Qt interface. We rewrite our code adapted to that, and it seems to work fine (at least in our tests).
I'm afraid we cannot do much more with previous Max versions.
In any case, remember that you create your own toolbar adding same actions (from the Customize Interface menu). Those should work well, because user's toolbar are slightly different.