Users who have had trouble with cancelling the subscription have started a subscription directly with Fabulous and bypasses Apple's ability to manage the subscription. Fabulous offers 2 subscription types, one can be made through an in-app purchase and can be cancelled by Apple. These charges are seen on your card as apple.com/bill to identify Apple as the merchant.
The other more common subscription is made directly with Fabulous and you can find out how to cancel those subscriptions with them along with more information here:
https://help.thefabulous.co/en/support/solutions/articles/101000545363-managing-a-subscription-created-via-fabulous
Just because a company has an app does not mean that Apple manages the subscriptions they provide. The only subscriptions Apple is able to manage are those made with an in-app purchase. Many other companies have apps and offer subscriptions that cannot be cancelled by Apple, such as Netflix, YouTube, Walmart, Uber, HBO, Amazon, and many more.