I can see subtasks in Backlog of my multi-project board.

[Zbigniew Kowalski]

In my company-managed project I have a multi-project board and Backlog.

Subtasks are visible alongside other tasks and stories when subtask`s parent is in sprint. It started to happen few days ago, previously subtasks  were visible only as an expandable option of a task.

I kind of expected for expandable functionality to work in sprint list but no such luck.

Can this be fixed?

Either hide "orphaned" subtasks (you can`t move them anyway) or show them in sprint list, as an expandable option, where their parent is.

[Ryan Brailey]

Is it possible to provide a screenshot with data redacted, showing the symptom? 

I just tried to replicate this in a test environment but sub-tasks will still show as expandable under the parent, as you say. On a default instance, proceeding to try and remove the Parent from the sub-task renders error "You can not remove a subtask's parent". 

If you scroll to the very bottom of the Backlog page and use the in-line option to create a new Issue, what are the Issue Types available?