When you leave some matter up in the air, you leave it undecided.
Example: My father can’t take decisions very quickly. He leaves matters up in the air for months. It drives my mother crazy.
‘Leave something hanging in the air’.
‘Leave something hanging in the air’ has the same meaning as `leave something up in the air’.
Examples: My boss, on the other hand, likes people to take quick decisions. Employees are discouraged from leaving something hanging in the air * Take a decision now. I don’t want you to leave it hanging in the air .