A higher CYA will just tie up more of the free chlorine but you would still be able to measure it so this is not likely the problem with the vanishing chlorine.

Yes, the clear cover will allow more sunlight through than the blue cover so it will cause more chlorine breakdown than before but it sounds to me like there's more than this that's causing such high chlorine usage. I'm guessing if you're using a solar cover you are not in a hot environment yet.

One test you could try is to add the chlorine after sundown and measure the level. Measure it again in the morning and see where it's at. That should help identify if you're losing it to sunlight or something in the water.

Peter