Looking at the schematic, I'd suspect C206, a 220uF, 90v
electrolytic that filters the supply controlling the
cathode voltage.
It looks like this value was chosen high enough to keep the
cathode biased above the grid for long enough to prevent
the un-scanned spot from appearing.
There are other possibilities, but that's what I'd try first.
The simplest thing to do is to substitute another known-good
capacitor of at least 200uF and at least 90v rating. If that
cures the problem, then replace the capacitor. (Bridging the
old cap with the new one might work, too, but there's a chance
that it's electrically leaky, which would interfere with bridging.)