You can’t really manage subscriptions in sandbox, but as Jean-Paul de Ville de Goyet found out on Apple Developer Forums:
1 month subscriptions auto-renew every 5 minutes. So far so good. They
auto-renew 5 times and then they stop, so after 25 minutes you’ll get
the 21006 error. However even when repurchasing the same subscription
it will NOT auto-renew again on the same test account since it has
already auto-renewed 5 times. So if you want to test renewal and you
have been messing with these subscriptions for a while you need to
create a new itunes connect test user. This is very annoying honestly
and it would be a lot easier if we could just reset the entire
purchase history of a test user account.
I tested my subscriptions the same way.