Bug report: Key signature changes

Sorry to be bothering all of you, but I have come to report a bug.

Key signature changes sometimes aren't "right". What I mean is, for example---

If I were to change the key signature from A major (3 sharps) to C major (no sharps), the C major key signature would have three natruals instead of being blank, to cancel out the previous sharps in the previous key sig. Sometimes it does this, sometimes it doesn't, yet when I do a playback, the music plays back just fine.

So is it just my problem or is this a known bug?