I also think it is the magnetism,
but turning the high volt cable a few degrees could help, there is conductive coating all over the inside, the high volt cable is seated inside the tube with 2 metal wires by transport the wire trembles and coating could get damaged having one wire not touching the coating making the flow of electricity under certain frequency's flow not completely all over the conductive coating. hence the dead spot.
personaly i also think it is either the magnetic coil, or a wire running to close to the tube.
You will not damage your tube by turning the high voltage cable, i worked on tv's for 20 years, if there are image probs after transport, that is the first thing you do.