Boards are not containers for issues, projects are. A board is a view of a set of issues you choose to include. The board filter defines what is on it.
It can look like a "move" if you have boards with contrary filters - for example, if you have board 1 with a filter of "issue type = bug" and board 2 with a filter of "issue type != bug", then an issue will appear to "move from one board to another" if you change it's issue type from bug to others, or back in the other direction.
But the issue is not moving. You're just changing the data that lets the board select it.
Have a look at the two board's filters to see what you need to change to get it to appear on one board and vanish from another.
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