Sorry - Google translate.☺️
The display is something like an old digital watch(no backlight). When the watch was low on battery, the numbers(text) were not black, but lighter (gray). This means that the power to the display was lower. The numbers were then legible when viewed at an angle (tilt). However, the minidisk has "constant" battery voltage and nothing changes when I connect the charger.
So I don't think there is a problem with the display, but its power supply.