Certain "Polar Coordinates Video" not marking as watched (or even started)

Hi, 

I'm working through multivariable calculus and there's one video that isn't being marked as watched (or even completed). It will say I've earned the 850 energy points and change colour, but when I refresh the page or try from a different browser it disappears. 

I've tried this on two seperate browsers (Chrome and FF) and private on each and can't get it to work. 

Video in question is: https://www.khanacademy.org/math/multivariable-calculus/integrating-multivariable-functions/double-integrals-a/v/polar-coordinates-1

All the other videos around it mark as watched so I suspect it's an issue with just this one (as opposed to me).