On a fresh Jira project, configured with Scrum, when trying to define the first sprint every user stories we have are correctly added but tasks, even if they are shown in the sprint after the drag and drop (or other ways of moving it into the sprint) AND a notification mail has been received to indicate the change, stay in the backlog.
The sprint context is already Global (no change needed) and I did not find any réindexation option as told on some resource we found.