Drilldown table doesn't work properly

Hi

I made a Drilldown Table and customized it. But the are two problems:

1. As I understood, the property HasChildren (from API /Children) is the key indicator to decide if an expand is possible or not. But my Children Query doesn't target the Children-API, it targets the AllWorklogs-API. Now, I have the problem when there are no children work items assigned, I have no expand-button even if the are worklogs which I'm targeting.

2. The export-button doesn't work. It shows up correctly but when I click on it, nothing happens. The exports on other reports working fine.

Here my definition of the report for better understanding:

{"type":"HierarchyTable","id":"","title":"PBI / Bugs (advanced)","subTitle":null,"w":8,"h":4,"odataVersion":"v3.2","endpoint":"workItemsHierarchy","query":"$select=System_Id,System_WorkItemType,System_TeamProject,System_Title,TrackedTotal,TrackedItself,System_State,System_AreaPath,System_AssignedTo,HasChildren,CustomStringField1&customFields=Custom.BillType&$filter=TrackedTotal gt 0&workItemsFilter=System_WorkItemType eq 'Product Backlog Item' or System_WorkItemType eq 'Bug'&$orderby=TrackedTotal desc","hierarchyField":"System_Id","childrenEndpoint":"workItemsHierarchy","childrenQuery":"({hierarchyField})/AllWorklogs?$select=Comment,PeriodLength,Timestamp,IsBillable,ActivityType/Name,User/Name&worklogsFilter=PeriodLength gt 0&$orderby=Timestamp","hierarchyType":"workItems","columns":[{"field":"System_Id","title":"ID","formatter":"WorkItemId","width":150,"expand":true,"child":{"field":"Timestamp","formatter":"CustomDateTime","format":"DD.MM.YYYY"}},{"field":"System_Title","title":"Title","formatter":"","width":-100,"child":{"field":"Comment","formatter":"String"}},{"field":"System_WorkItemType","title":"WorkItemType","formatter":"","width":100,"child":{"field":"ActivityType.Name","formatter":"String"}},{"field":"TrackedTotal","title":"Tracked","width":100,"formatter":"TimeLength","child":{"field":"PeriodLength","formatter":"TimeLength"}},{"field":"CustomStringField1","title":"Bill Type","width":100,"formatter":"String","child":{"field":"IsBillable"}},{"field":"System_TeamProject","title":"Project","formatter":"","width":100,"child":{"field":"User.Name","formatter":"String"}}],"autoSubtitle":false,"timeframeFilter":null,"y":0,"x":0}

Did I make a mistake somewhere?