Thanks for the reply
I simulated two cases in differential via with back drill and without back drill also taking ground vias into effect, below is the geometry that i took into consideration ground vias have the connectivity in LAYER (2,4,6,9,11,15).
Differential vias traces entry layer is TOP and exit layer is 10, so extra stub is created from layer 11 to layer 15.
While simulating return and insertion loss profile in touchstone viewer, return loss and insertion loss is more in case of backdrill as compared to non backdrill, it should not happen.
With backdrill those ground vias are still from TOP to BOTTOM those are not backdrilled, my point of view is whether it is having larger return current loop from layer 11 to layer 15, I know that it will go to shortest return path.
whereas without backdrill differential vias and ground vias are adjacent to each other means return current loop will be less.
Do those ground vias are creating that change in insertion and return loss profile