I think you're describing a bug that's triggered by multiple leading values—specifically, applying a global leading value (i.e., to a text box) after having applied a more specific, local value (i.e., to a line or paragraph). It should clear up if you restart the application.
Thank you so much.
What a rubbish bug.
A nice addition to Fireworks CS6.
Seems to be one the very few new features.
I appreciate you taking the time to answer Groove.