I am not sure if I read the screenshot correctly (I normally use the "old technical log"), but it looks little bit like the "header work item" (the first item of the workflow) gets completed by function call to SAP_WAPI_SET_WORKITEM_STATUS. Now I am not even sure that is it possible to set the whole workflow completed without completing/logically deleting the child work items. But let's say that somehow the header work item gets completed by the function, and this does not take care of the child items. If this is true, then this explains the strange behavior. Then you should investigate that from where this function gets called - is it custom development, and if it is, then it is perhaps developed incorrectly.
Kind regards,
Karri