Interesting observation. My main bass had a [i]slight[/i] dead spot at the 7th fret on the G string. No biggie, but noticeable reduction in sustain compared with every other note.
Then last night I noticed that the neck bolts were slightly loose, so I tightened them up, and the dead spot has now moved to the 5th fret!
I can only assume that the tighter join has shifted the point at which the resonant frequency is cancelled out.