Paul formulated on Wednesday :
"Pure flyback single-transistor inverter, makes voltage spikes
in the kilovolt range in idling condition, so that the lamp
will start. Disadvantage: One filament evaporates and blackens
the lamp, makes it electrically unsymmetric and shortens the lifetime."

Basically a pulsing DC. The fix for the the black at one end, was to
simply take the tube out and reinsert it swapped end for end.