Calculating a variance

If you want to get the difference between 2018 and 2017 and since you have already created external measures, you could simply write  Variance = [2018 Issued] - [2017 Issued]

If you placed the measure you just created in a pivot table and divide them into columns of years, this is what happens

• 2018  = [2018 Issued] - [2017 Issued] > This returns the value of [2018 issued] since this column is for 2018. What's visible in the current filter context is just 2018. 2017 is blank.  So  [2018 Issued] - BLANK() = [2018 Issued]
• 2017  = [2018 Issued] - [2017 Issued] > This returns a blank value for [2018 issued] since this column is for 2017 . What's visible in the current filter context is just 2017. So BLANK() - [2017 Issued] = negative of whatever the 2017 value is. 

I think what you are trying to achieve is the year over year difference and not simply the difference between those two years. If so, this would have involved a Time Intelligence function if you had a standard data column but still a workaround is possible.

Here's the modified measure to be formatted as percentage

Variance YoY% =
VAR YEAR_ =
 SELECTEDVALUE ( 'Table'[Year] )
VAR LY =
 CALCULATE ( [Sum of Issued], ALL ( 'Table'[Year] ), 'Table'[Year] = YEAR_ - 1 )
RETURN
 DIVIDE ( [Sum of Issued] - LY, LY )

A similar tutorial can be found on youtube along with standard time intelligence calculations. https://www.youtube.com/watch?v=g_3eLaKgeEQ