That is a disappointing and frustrating problem. There is a piece of polarized plastic over the display, which blocks light waves except from certain directions. Your old plastic starts losing it's clarity(no decent UV inhibitors back then) and starts catching and passing light from other directions. It is hard to find the correct sheet, as there is an optimal relationship between how far apart the microscopic lines in the polarized plastic are, and the distance the plastic is from the actual liquid crystals. And the direction the lines run also is a factor - and different companies do things differently to avoid patent issues.
The plastic is often hard to remove, and it seems unless you are a manufactuerer of smaller LCD displays, the correct polarizing film is virtually impossible to find.
Early Seikos and CItizens (just my opinion) seem a bit more prone to this than some others. Possibly they were using same suppliers for the film or the entire display matrixs.

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