I’m a bit confused. If you were charged, that means the trial should’ve been over right? Did you cancel before the trial ended and before you were charged?
If you canceled early to avoid being charged, then the trial ends at that time as well. The trick is to cancel right before the trial ends.
This is normal with Apple subscriptions. With many apps if you cancel you will still have the remainder of the free trial but not with Apple subscriptions. There is a little pop up saying this and asking if you want to do it, but most people don’t read that.