Same. My subscription has been renewed 14 times and its not possible to cancel it. If I try to manage the sandbox account in the AppStore on my device I get a "Can not connect" error message. VERY FRUSTRATING.
I've created new sandbox accounts but everytime I try to make a new purchase on the new account I get the 'you are already a subscriber' message.
COME ON.
Contacting apple through developer.apple.com is a waste of time. They dont understand me or what the issue is. They just tell me to create a new sandbox account and then after that you are ignored.
I believe its an issue with using your real account as a sandbox account. Apple states that there is no sandbox account connected to my real account but there clearly is because I can sign in as a sandbox account in the AppStore on my device.
.. So if you used a 'real' sandbox account from the start this probably wouldnt have been an issue but now im STUCK and noone can help me.