This is definitely a bug within NextGen.
Create any custom filter for the sprint board.
If a task meets the criteria AND it has sub-tasks, it won't show up on the sprint board.
This is a bug because tickets that DON'T have sub-tasks will show up and surprisingly, the ticket with sub-tasks will show up when you're looking at the "backlog" in the "current sprint".
is there a workaround for this?