Line chart label formatting acts differently for the same measure as a master measure

[smithc2005]

This measure (below) causes the Line Chart Y-Axis label to display differently when the measure exists as a master measure. 

The measure text is : 
NUM(SUM( {$<$(vSalesSACurrent)>} FIN_GP)/
SUM( {$<$(vSalesSACurrent)>} UNIT_SELL_PRICE_EXT), '#,##0.0%')

The variable vSalesSACurrent is defined in the load script as:

LET vSalesSACurrent = REPLACE('[DATE.autoCalendar.Year]={|(vCompareYear)}, [InBDYTD]={"|(vInBDYTD)"}, [InBDMTD]={"|(vInBDMTD)"}, [COUNTRY_CD]={|(vCountry)}', '|',chr(36));

As a master measure, with "measure expression" selected as the number formatting option, the Y-Axis label is displayed as this:

Simply unlinking the measure from the master item causes it to display the way I would like:

What could possibly cause this?