A few sets had a "spot killer" switch built into the on off switch but
Zenith never did. Its part of the charm of an old TV ! Check the
G-1 CRT voltage, there may be something off making it
worse. Other than that the HV will stay charged & cathode stay
hot & emit electrons. Probably a switch to ground out the G-1
at turn off may stop it. But it is normal. Even a lot of color sets
will do it. Growing up we had about a 21" Zenith in our room. With
the lights out that thing would glow for hours although it would
spread out & slowly dim.
73 Zeno
LFOD !