The current shadow color is Shade Black, which is also used as the ViewBackground color in Breeze dark. This makes the shadow invisible when 2 windows without borders overlap with touching View areas. It also makes color banding worse on cheap displays. Changing the color to black makes the shadow visible on most backgrounds and slightly improves the look of the shadow on cheap displays.
I changed this from black to Shade Black as a compromise to get https://cgit.kde.org/breeze.git/commit/?id=9fb5d3abb18cde0ef63102cb06f1a936152ccaef done and I think we have to consider the experiment a failure. It results in various visual regressions and black is a better default.