I had this in my old flat, created by weathered (cheap) external rendering, thus making is permeable
It was difficult to diagnose, they spend a while inspecting the flat roof and gutters. It was probably a multitude of problems but they identified this as the main cause for ingress
Yep, we've had this. Water literally dripping though the top of a window due to blown render outside (combined with a clogged gutter chucking water at the wall).