Mis-aligned electron guns? If the gun assembly got slightly twisted, or too close or distant, when they were reattaching the neck of the new guns to the CRT tube? I don't know what tests Hawkeye does after a rebuild, presumably more than an emission of the cathode test...
Can the metal parts of the electron gun get magnetized? Maybe a degaussing of the neck might help.
Would the deflection yoke being too close or distant (front to back) do this?
If it's the mask, maybe it can be coaxed into position with a magnet? But you'll need to degauss afterward.