This can only be fixed with DX12 I learned. The issue is this: when a part of the plane blocks part of, say, a building, then the shadow of that building cannot be drawn at that moment. Only when the whole object is in view, the shadow gets drawn completely. Seems to be an issue in any game without ray tracing.
Pop in is a different thing, so I learned too. This is a lod-related issue, like complete buildings coming into view too late.
I hope that I’m summarizing correctly here.