I know these sets pretty well--worked on a lot of them when new and later--one of two possible ideas=
1. Either a odd convergence problem==confirmed or denied by going into the convergence menu (in customer menu) and selecting advanced which will bring up a grid of lines--if see the same issue (I will attach a photo of a Mitsubishi with this problem) it can be fixed.
2. Some sort of issue with the circuit board on the end of the Blue tube(you are missing blue=black) but usually in this set if that happens it is uniform and on both sides of the screen, not the top.
Let me know what you find.
I doubt it is convergence, but does not hurt to look--if the missing area was perfectly rectangular I would suspect up/down centering adjustment.
There is one other possible thing to try--we will leave it for last--set has 3 hidden menus and one is OPTION menu which allows a hard reset of entire set--sets everything back to factory except convergence adjustments which are stored in another area.