Not sure how you canceled your Stan subscription.
Canceled the subscription via Apple or via Stan?
Normally you will receive a confirmation afterwards.
My Stan subscription expired in February this year.
See -> View your purchase history for the App Store, iTunes Store, and other Apple media services
View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support
- a subscription that you have canceled or expired, will disappear from your purchases after about a year.
- so that Stan canceled or expired subscription should still be in your purchase history.
Do you still have that confirmation (check your mail box), use it as proof.
See -> https://help.stan.com.au/hc/en-us/articles/202759790-How-do-I-cancel-my-Stan-account-
See also the following Apple articles:
If you want to cancel a subscription from Apple
If you want to cancel a subscription from Apple - Apple Support
If the subscription that you want to cancel is not listed
If the subscription that you want to cancel is not listed – Apple Support (UK)
How is it possible the it is expired but for Stan to continue to charge my credit card every month?
Contact your bank, depending on your bank, they can collect the amount back within 30 days.
Ask your bank to block Stan from taking money from your account.