Returning to multiple choice question causes answers to double-highlight

Hi all,

  I've noticed this issue in all of my recent projects. I have a multiple choice question set up, all is looking good.

But, if I advance to the next slide and return to this one (go forwards and backwards one slide), the options appear to have a second highlight overlayed.

Is there any way that I can fix this? Does it have something to do with the master slides? Or is it just a bug that I can't really control? All advice is appreciated!