Cannot add Date in my matrix as row without Hierachy

Hello Guys, 

I have a strange case. 

I have already added a date field as row called "Posting Date" which is correctly displayed 

Now I want to change the field from "Posting Date" to Document_date

They are the same datatype, coming from the same table but I cant insert the date without hierachy. If I remove the hierachy its delete the document_date row completly 

the same happens if I remove Posting_Date and Insert Posting_Date again. Its also impossible to remove the hierachy part. 

What happend and How can I insert my row like i did before ? 

Thanks in Advance 🙂 

This is an example of the current Posting_Date row