You should be able to cancel from the store app (App Store / Play Store) directly.

After starting a subscription I use Launcher.OpenAsync("https://apps.apple.com/account/subscriptions"); to open the on-device subscription window, but no subscriptions are shown there on my test device.

Is this using Sandbox testing? Sandbox subscriptions don’t show up in your regular App Store account. You can find the subscription management page for the Sandbox account in the Settings app under App Store → Sandbox Account (all the way at the bottom). When you tap on that, you get a dialog with the option “manage”. You can cancel the subscription from there.

From my experience you will not get ‘Cancel subscription’ button here App Store → Sandbox Account in case if you conduct subscription in app. Cancel subscription button appears only if subscription was done inside sandbox manage list. This could help: https://stackoverflow.com/a/73269856

for my case it always show as expired event i via flutter revenuecat sdk i can see that use is subscribed and has expiration date in the future however in appstore->sandbox account setting expiration date is in the past