What windows accent color are you using by chance? I have mine set to a light lavender, and there is very little contrast between it and the slider background, so it can be very hard for me to see the level indication. (not sure if you can set the accent color to a gray, but I could see an oversight where you could set it to a very similar color to the indicator bar)
Note: I just tested, and it seems the “Storm” accent color is just about the same gray as the indicator bar, this could be your issue