CALCULATE / FILTER DAX code issue

Hi SJHardeman 

First, here's a bit of an explanation of what's going on:

1. When a measure such as [Latest month] is invoked, it is automatically surrounded by a hidden CALCULATE (...). 
2. The FILTER function is an iterator which evaluates the condition in the 2nd argument in the row context of each row the table specified in the 1st argument.
3. Within a row context, CALCULATE triggers context transition, which transforms the row context into an equivalent filter context. All columns within the "current row" become filters.
4. Applying this to your measure: [Latest month] and [First month] are evaluated within a filter context corresponding to each row of the Data table. This means the conditions [Latest month]='Data'[Period] and [First month] = 'Data'[Period] are always TRUE.

Here is a good article on this overall topic:

https://www.sqlbi.com/articles/understanding-context-transition-in-dax/

A couple of side points:

1. FIRSTDATE and LASTDATE return tables (1 row x 1 column), and are typically used as SetFilter arguments within CALCULATE. If you just want to return a scalar value, you can use MIN or MAX.
2. For any date-based filtering, it is best to create a separate 'Date' dimension. However, I won't worry about this for the purpose of this question.

You can fix this a few ways. I would personally recommend creating these measures:

FTE Sum = 
SUM ( data[FTE] )

FTE First Month = 
CALCULATE(
    [FTE Sum],
    FIRSTDATE ( data[Period] )
)

FTE Last Month = 
CALCULATE(
    [FTE Sum],
    LASTDATE ( data[Period] )
)

FTE_change = 
[FTE Last Month] - [FTE First Month]

Does this work for you?

Regards