I am not excluding the fact that it could be a bug but first let see if the regular behavior of the plugin is applied.

When you switch to a language the menu should show only the items from the same language.

Is the translation url set for the “disappearing items”?

Alternatively your can try to disable this filter by checking the “Disable Menus Query Filter” box in admin panel > Languages > Settings

Let me know if it works for you.