Power Pivot Sort By Column Not Working in Excel 2016

I'm trying to sort a Month value (e.g. Dec-16) in a date dimension using a Month Sort column. I can set the sort by column in the data model no problem but when I use the Month value in a pivot table it is sorting alphabetically

• The month sort column is of data type & format a whole number
• The date dimension is set as a date table
• The cardinality between month value & month sort is 1:1
• I have refreshed the model & pivot table
• I have fiddled around with the sorting of Month Value (A>Z,Z>A etc.)
• I'm using the latest build of Excel 2016 (7571.2072)

I've done this hundreds of times in SSAS Tabular & Power BI but not recently in PowerPivot. What could I be missing?

Thanks!