We could not reproduce the issue in internal Agile studio 8.1 instance. Can you please check the following
1) Please confirm that story in the BL is not associated with sprint
2) Please launch tracer and open bulk processing window and see if you get any error. You can specifically trace PreBulkProcessing activity in PegaProjMgmt-Work-UserStory class. Please share error if any
3) Open RD OpenBLStoriesForBulk and run it by setting BacklogID as BL-5(Private edit-> In Edit filter section replace Param.ParentID with BL-5 ->save and run) . It should give the list of stories for the BL
Also please confirm if this issue is happening only with one BL or is it issue with all Backlogs.