Angular Web Portal - Issues with attestation grouping

Hello IM Community,

I've tried getting to know the angular development side of OIM. 

Earlier today i hit somewhat of a roadblock when i noticed that the current angular project in Github, seems to have a slight issue, which in cant seem to solve. 
Whenever i try to group Attestation Cases, after compliling the project and importing it, i seem to get the expandable elements, but the entries arend grouped by these. 

Could anyone point me to, where id have to fix the issue?

I've already checked the ATT-Project and also looked into the QBM-Project and looked around the data-table.components, but couldnt find an issue. 


In kind regard, 
Daniel