This is not something that should work differently in the current version. When you spawn meshes before activating the GridManager I see no reason why the GridManager should treat these meshes any differently than ones that were placed before launching the game.
I just tested this out myself. I made a new actor that simply consists of a scene component and a static mesh with collision set to block WallTrace. I spawn this actor right before activating the grid manager (without a delay), and it affects walkability just as if I had placed it beforehand (I have TraceForWalls enabled in BP_GridManager). Could you tell me about how your SpawnBuilding function works?