PicsArt Still Charged Me $17 Even After I Canceled During the Trial

I had purchased the PicsArt app annual plan for $17 through the App Store on my iPhone and started using it with a trial period. Before the trial period ended, I went into the subscriptions section on the App Store and pressed the "cancel subscription" button, believing I had ended my subscription this way.
Despite this, today, on June 4th, I saw that a PicsArt annual membership fee of $17 was charged from my account. I've been put in a difficult position because my subscription continued and a charge was made, even though I had canceled my trial in time.
I am requesting a full refund of the $17 charged, from both PicsArt and the relevant App Store department.