TFS I am facing bug - in Backlog for some user stories the iteration path changed to default iteration without any action from my side

I skim through backlog, and navigate through sprints open stories (view only)

After I complete the review for sprint user stories, I found that the system change iteration path for some stories (even these stories I did't access) to backlog default iteration path

would you please advise with solution or workaround for this issue 

Many thanks ..