It seems that if you move an event with time zone information from one Google calendar to another, the time zone info is lost. But if you move it back to the original calendar again, the time zone info reappears! I am guessing what is happening is something like:
ADD TIME ZONE XYZ TO EVENT AAA MOVE EVENT AAA TO CALENDAR ABC UNDO MOVE EVENT AAA TO CALENDAR ABC
and the last two commands cancel each other out. Does anyone know?