171 appears to be the catch all gate they (the app/website) assign a few days out to a bunch of flights. I've seen it before for yyz-yyc and yyz-yvr. I've only seen it 2 or 3 days before my flight either backtracking through the app(both old and new) on the track inbound option or on the website under flight status. It invariably switches to a domestic gate roughly 30 hours before departure. Never had it on a boarding pass though so maybe this is an enhancement I haven't had the chance to experience yet.