When I drag a file to the Desktop folder widget I notice the following: for a short period of time there is instead of the green (+) icon the red (/) "not possible" icon. The time is long enough that when I immediately drop the file (which happens regularly if I do the drag and drop in a smooth motion) nothing happens and I have to start over again. There is way to reproduce the behaviour: 1. Drag a file with a quick motion to the desktop and then abruptly stop the motion. 2. The red (/) icon is visible. 3. If the mouse is now moved a tiny bit it changes to the green (+) icon. 4. If the mouse it not moved the red (/) icon remains and releasing the mouse button to drop results in no action.
Works fine in Plasma 5.17 and Qt 5.13. IIRC there was a Qt issue that regressed this, but it's been fixed now.