Now I see that the problem is on other level. Here is my test file.
I iterate all artboard groups/layers/symbol_instances to get layer frames. To go inside a symbol instance I collect symbol overrides and process layer.master.layers array.
It worked fine before Smart Layout. But now I can't get a valid frame for "Screen Title + Breadcrumb Instance CUSTOM" / "Breadcrumb Instance" / "Link - Breadcrumbs" hotspot, because layer.master.layers provide me original master childs, without Smart Layout transformation applied.