Having a subscription doesn't mean zero queues. The plan is called Priority, which means priority over the free users.
GFN doesn't have unlimited rigs. So, when game releases a new hyped season/update and/or GFN adds a hyped game, you might face queues due to the amount of players trying to join at the same time.
It's informed in the ToS.
Despite it's a rare thing, you should be aware now.