StDev discrepancy when using similar tables

[TMO_KY]

I wasn't exactly sure how to word my problem to make it stand out from similar but different issues.  Anyway, I have a DAX table that I created and it works flawlessly but I was recently asked if I could add an additional column, a unique ID, to the table but when I did, it throws my StDev off, therefore throwing off my Z-Score and I'm not sure how to fix it.
Below is the Expected vs Incorrect output columns.

The measure I'm using for the Expected Output visual is as follows:

StDev_24_TEST = 
CALCULATE(STDEV.P('24MonthTable'[Failures]),
               DATESINPERIOD('24MonthTable'[Month-Year],
               LASTDATE('24MonthTable'[Month-Year])-1,
               -24, Month
          )
     )

 
I tried using the same measure for my "duplicate" table but it returned all 0's.  I tried several different methods and all but one variation returned 0.0.  The measure that "somewhat" worked, and I use that term very loosely was:

STD24 = 
VAR s =
    CALCULATE(
        STDEV.P('TEST'[Failures]),
        ALL('TEST'),                   -- Remove any other filters on 'TEST'
        DATESINPERIOD(
            'TEST'[Month-Year],
            LASTDATE('TEST'[Month-Year]) - 1,
            -24,
            MONTH
        )
    )
RETURN
    s

The ONLY difference between the tables is one column, [PR ID] which I was planning on using to link to other tables because it's a unique identifier.   

[lbendlin]

Please provide sample data (with sensitive information removed) that covers your issue or question completely, in a usable format (not as a screenshot). Leave out anything not related to the issue.
https://community.fabric.microsoft.com/t5/Community-Blog/How-to-provide-sample-data-in-the-Power-BI-...
Please show the expected outcome based on the sample data you provided.

https://community.fabric.microsoft.com/t5/Desktop/How-to-Get-Your-Question-Answered-Quickly/m-p/1447...

[TMO_KY]

Ive uploaded a pbix file with sample data to my dropbox.  Hope this helps.

StDev_SampleData.pbix 

[lbendlin]

In your TEST table each failure has its own row, therefore the standard deviation is computed across all these 1 values. You need to use STDEVX.P rather than STDEV.P 

Having said that I can only get half of it to work.

[TMO_KY]

That's more than where I was able to get so I appreciate it.  I'll have to talk to my end user and get their thoughts. if the Product Family will be in a visual or not.  I know it's used as a filter sometimes but not always.

What if there was a sum measure for the Failures in the test table and do the stdevx against a measure?

[lbendlin]

I think that should work, as long as that measure evaluates for each row of the table argument in STDEVX.P .  The trick here is to present a suitable table - which can also be a table variable.

[TMO_KY]

I appreciate the insight.....and I need to see what's going on, your suggestion worked for the data I provided (hard-coded in query) but once put in with live data, it throws different values, even when filtered, and all data types are the same.  Either way, I appreciate the help and quick responses.