This manifests are a couple bugs which might be all sharing the same root cause.

The easiest way to set it up is have a few labels in a series in a course section, some of which have short text and some which have long text.

In the left course index if you mouse over the links over the area where the text is shorter than the menu item then you get a hover effect which gives the expectation that they are clickable. You can actually click them but instead of navigating to them they only add the blue keyboard focus border.

ie in the gif below when you click on the right hand side of some links you get nav to that label and it highlights the item, on the shorter text ones you get the blue focus ring.

If you compare this to say the top level tab items then the regions for the links and the blue keyboard ring focus are one and the same.