The price of electricity is set by the marginal cost of the most expensive individual source - if your grid is 80% solar, 20% coal, the price you pay is the price of coal, because the solar providers can increase their prices to just below that of coal. Obviously I'm simplifying somewhat, but that's the general dynamic.

This is "by design" in the sense that it offers big subsidies to more solar generation to come online, but you won't see the biggest price cuts until the last expensive sources are pushed off the grid entirely. Because Germany's marginal source is coal, they pay way more than countries whose marginal source is gas or nuclear.