Shopify's app billing cycles
If the app has a free trial then the billing cycle begins at the end of the trial. If the app does not have a free trial then the billing cycle begins the first time a recurring app charge is approved. The billing cycle repeats every 30 days until the app is uninstalled.
Shopify generates recurring app charges on the first day of each app billing cycle and bills them with the next 30-day Shopify invoice. Because of this, if you uninstall an app after a charge has been generated but before your next Shopify bill the app charge will still appear on your next Shopify bill. This is not an error.
In the example below, the bill on May 5 includes the app subscription that overlaps from April 20 - May 20. The bill on June 4 similarly includes the app subscription from May 20 - June 19 (since May has 31 days). If the app was uninstalled on May 28 the charge that it generated on May 20 would appear on the June 4 bill.
For more information see Shopify's app subscriptions documentation.