Issue when using Calculated Columns as matrix header

Hey,

I'm busy recreating a timesheet from our previous reporting tool to Power BI and I had a weird issue.

In my matrix, I can add multiple levels. For this one I have 4 levels coming from a Date-Table:

- Month Name (Long)

- Week

- Day_Of_Week_Short (calculated column POST Query)

DAY_OF_WEEK_Short = LEFT(TimeFields[DAY_OF_WEEK];1) & LOWER(RIGHT(LEFT(TimeFields[DAY_OF_WEEK];2);1))

- Daynumer of Month

Working Layout 
However the simplest change already gives errors. I tried switching "WEEK" with "WEEK_NR". A calculated column that is defined as follows:

WEEK_NR = "Week " & TimeFields[WEEK]

But that gives the following, unsatisfying result:
Broken Report
I've checked the syntax of WEEK_NR but didn't find any errors.
Even weirder is that if I'm filtering our certain WEEK_NRs I get more data and kind of what I want!

Filtered Broken report

Is this a bug? What am I doing wrong? If the calculated column is doing so much damage, why is the "Day_Of_Week_Short" Not giving any troubles?

Additional Info:

- The value I'm using is in a different query to trhe dates-query. They're linked in a many to one relationship that cross-filters both ways.
- I've tried changing August to "August 2018" in a similar way and this resulted in a similar error.