The keyboard is apparently in some funky state until the first key is pressed. This manifests in that if the first key pressed is one that is missing from the key map, then that key press will be screwed up.
In detail, what happens is:
1. Xvnc gets a keysym it cannot find in the mapping.
2. The new keysym is added to a free keycode (usually 255).
3. The client gets an X11 event that the keyboard mapping has changed.
4. Xvnc sends the keycode that is freshly reconfigured.
At this point you'd expect the proper keysym, but instead the client sees NoSymbol.
The weirdness continues on the next missing keysym, where Xvnc will still consider the keycode free (so it will again assign something to e.g. 255).
The problem only exists for the first key press, so it is probably not something that will happen that often in practice.
This is not an error anyone has seen in ages. Assuming it got fixed somewhere along the line.