I have no good explanation for the phenomenon.
But if it were happening to me I'd be very concerned at the possibility that my body was proving a better ground source than the actual ground wire. Copper wire generally being a much better conductor than human flesh..
So, might be worthwhile to double check that connection.
Also, check the actual resistance between the input jack ground connector and the chassis. With a sensitive input even a few ohms there can be problematic. Some people recommend a small value capacitor from input ground to the chassis, especially if the grounding scheme places the chassis ground point far from the input.