Let’s say you have two JIRA projects, project K using a Kanban board and project S using a Scrum board. Each project with its own set of statuses and workflows.
Now, users of project S want to see issues from project K on their Scrum board. They update the board filter accordingly, but to their surprise, many project K issues are not visible in the Backlog.
Why? because a JIRA board (either Scrum or Kanban) only shows issues whose statuses are mapped to the board’s columns (e.g. To Do, In Progress, Done).
Issues with unmapped status are not displayed on the board at all. This is useful in some scenarios but confusing in others, such as in the example above.
To remedy the situation, go to the Scrum board’s configuration, and under the ‘Columns’ sections you can map or unmap statuses as you see fit.
Adventures in SCM 
Great point. Please note that the downside of this is that the more status types you have it is harder to move tasks between columns to the desired status. My suggestion is to check which tasks are hidden/not-shown and fix them.
I agree. However sometimes you are forced to have several status types due to external constraints (regulations, contractors, etc). In this case you’d want to have a strict workflow, so when dragging a task you will have only a couple of ‘drop target’ options.