I know exactly what’s happening and it was a mystery to me for a while as well.
When you slide a video event, it always snaps to a frame. However if you drag an audio event it can end up in the middle of a frame. What you are running into is that your audio which is connected to the event frame was moved somewhere between frames. When this happens your fade will disappear.
Look closely at your video, zoom in on it and see. The audio is probably not at a frame beginning or end but somewhere in between.
This can also mess up other things because all frames after that are probably don’t start at a frame but half way in between.
Roger Bansemer – PaintingAndTravel.com