From a certain nesting level of course categories, breadcrumb navigation re-appears in the course header again!
At least, if it is removed, let it be removed everywhere.
Note that in
Boost Union theme, a theme which looks and behaves 100% like Boost out-of-the-box, but allows for several customization, there is a toggle "Display the category breadcrumbs in the course header" which allows for a complete re-introduction of the course category breadcrumbs ("Yes"), complete removal of the course category breadcrumbs ("No") (i.e. already fixes the mentioned bug) or to "Change nothing" (Default, leave it as is).
Best,
Luca