Forgive me for speculating about a detail I don't understand.
Just how how is "slippage" detected ?
While I understand that some low-end AWD models (The Briton?) don't have ABS or even rear disc brakes, I have always assumed that those "gear teeth" tone rings on the CV axles were used to generate some sort of a "pulse" signal that gets compared between front and rear, and when there is an imbalance between the front and rear axle pulse signals, the electronically controlled clutch packs are activated.
Perhaps on your car there are no tone rings on the front CV axles or the tone rings are so badly rusted and/or corroded that they aren't generating a proper "pulse" signal ?
Maybe I am totally off-base here and Subaru uses an entirely different technique for measuring axle rotation ?