Missing values from calculated column

Can anyone help me with the problem i'm facing.

I have a simple calculation view as shown below.

In the join I add one calculated column to my dataset.

MEDIATYPE with a hardcoded value 'Call'

When I preview the result of my view it looks ok, so far so good.

But when I use the distinct values in this same data preview it gives a result I did not expected.

0 values found

Also when I open this view in SAP Lumira I get a empty column

Can anyone tell me what i'm doing wrong or how I can fix my problem.

Thx