Results show I can have beer (green) but yeast is marked red. Is the yeast in beer somehow acting differently to make it show in green?

Yes, it can. Yeast is also found in bread and it can react differently. We take each item as a whole and test it to the volume of 100%. Beer does contain yeast but perhaps to the ratio of between 2 - 15%. As such, it can be quite useful to use a rotation and variation diet, meaning that it may also be possible that if the beer was overconsumption to the consumption of 100% yeast - it may instigate a reaction.