Actually its not a bug in DevTools. It seems to be bug with logging.
I've added some more code to test it after this line:
let layer = importer.importAsLayer();
let layer2 = sketch.fromNative(layer);
let doc = context.api().selectedDocument;
let parent = doc.selectedPage._object;
So layer represents native object, layer2 API object.
First it logs layer right after importing SVG, then script places layer into current document and logs it again.
Seems to be error with logging when layer has no parent layer or something like that.