A board selects a set of issues. If you create a new issue, it needs to match the criteria for that board. I suspect your board says something like "project = X" and you've created the issue in project Y, so it's not in the backlog. Or "project = X and status != new" and the new issue is in status new. Or... well, there's loads. You can go to the board configuration and view the criteria there.
It could also be that the imported issues are in an unmapped status. Ensure that the issues status exists on the board.
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