With the new symmetrical window shadows in 5.12, it is noticeable that although three sides of the window have a shadow, the top does not. I can't tell whether this is intentional or not.
Shadows are provided by breeze. Shadows have some offset on the top https://github.com/KDE/breeze/blob/master/kdecoration/breezedecoration.cpp#L711 So, that's intentional.
* and they have some offset on the top: Sorry, I'm writing from mobile. :)