Question for Ontarians about preferred insurance vendors

Sorry that this is only relevant to those of you in Ontario, Canada, but your help would be much appreciated. My partner's insurance coverage changed recently. His provider (Sunlife, but through something called PeopleCorp?) now has a list of preferred vendors for prescription drug coverage. Shoppers Drug Mart is not one of these vendors. Sooooooo annoying.

I also have my own coverage through Sunlife (80%), so I thought his would still kick in as secondary coverage for the remaining 20% if I continued filling my prescriptions at Shoppers. However, that doesn't seem to be the case.

I'm going to call Sunlife to see if I can change this, but I thought I'd check here first to see if anyone has any experience with this. Thanks!