As Zed has mentioned in a certain previous Post, breadcrumbs are not working in pages in Mantra theme.
Just in case I inform: also Tempera has the same problem. (Parabola is OK).
Breadcrumbs can be see in Mantra and Tempera if a page is with DEFAULT LAYOUT. Otherwise they just disappear.
I hope, when You will have time, You can improve them in both themes!
With best thanks,