I'm unable to open the file due to the version of Tableau...but is it possible that the dimension you're using to color doesn't exist in the prior year, and so it's breaking the reference line? Haven't tested it but that is my first thought.
Thanks for responding Joe.
I think you're right. When the current year dimension value I'm using to colour does not match the prior year value then it doesn't work.
This suggests that the LOOKUP() function is operating over month by dimension. This isn't what I'm after. I just want to return the prior year value regardless of the value of the colouring dimension, and to then decorate the bar according to that value.