context-border is added with negative margin to surround the highlighted element, in certain situations where there is no physical space in the design to allow for the border (possibly where overflow: hidden is used for instance) the border on one side may be invisible.

It's possible this could be fixed with z-index, but IIRC D7 simply adds a border *inside* the highlight element and I suspect there is a good reason for that. I'd suggest a move to D7 style contextual-borders.

Patch to follow when I find time :)