Strange, that should work. Usually issues like this happen from a double-store problem somewhere. Are you using CB Auto Actions to act on any CBSubs triggers? That's usually where the conflict can come from.

Additionally is there a reason you're needing to do this? If it's for searching purposes the CBSubs Plan Image integration which provides a plan image field to represent a users subscriptions has built in support for searching so it can be used to search userlists by plan.