It does appear to be the LCD, it's a VFD actually, but with the same controller so LCDOUT commands are used.

I am able to confirm everything is fine if I do not update any custom characters, but can use them if they are defined
at the start of the program and then not changed, so no scroll effect

One interesting thing I learned is that if a custom character is being displayed, and then you change it,
the display does not need to be updated, the character changes instantly on the display.