Formatting not working properly

I've a simple table with a date field called "ServDate" formatted as a date *3/14/2001 with values ranging from 9/4/2018 to 3/29/2019.   The table has ~134K rows. Creating a new column like this

MMM = MONTH(a[ServDate]) 

I get: 

ServDate                      MMM
10/2/2018                    10
10/2/2018                    10
10/18/2018                  10
... so this works. 

But IF, we add this piece, MMM = FORMAT(MONTH(a[ServDate]),"MMM"),  we get 

ServDate                      MMM
10/2/2018                    Jan
10/2/2018                    Jan
10/18/2018                  Jan

The month here is October, not January. Please let me know if you want the pbix.