Why was I charged for a canceled app subscription in April?

What is the exact description of the charge you see on your billing statement. That will help determine if it is a charged managed by Apple or billed directly from the developer.

Just for reference, the subscriptions managed by Apple are available here:

View or cancel app subscriptions on iPhone - Apple Support

Along with the Purchase History from Apple managed purchases:

View your purchase history for the App Store and other Apple media services - Apple Support

That description would indicate it was made with NFL directly and not an in-app purchase that is managed by Apple. That explains why it does not appear in your purchases and an Apple charge would appear as something like apple.com/bill.

They offer the subscription on their website, Roku channels, and even apps on smart TV's. To cancel a subscription made directly with NFL.com, follow the steps provided below. I have no explanation why you would not be charged last month, but you can use support on their webpage for more information about the subscription.

1. Visit NFL.com and sign in, or go to the Account page if you are already signed in.
2. Click the arrow next to NFL Subscriptions to Manage your subscription.
3. Within the Manage Subscriptions section, click the arrow next to Manage
4. Click the Cancel button 
5. You’ll get a message saying We’re sorry to see you go. Please select a reason why you’re leaving and click Cancel.           
6. Your subscription will be cancelled and you’ll be taken to a page that reads Miss you already. Click Back to My Account to return to your account settings.

Hello!

You can try requesting a refund here: Request a refund for apps or content that you bought from Apple - Apple Support

Have a great day!