If anyone is still confused with this, make sure you check the ORDER of your subscriptions within your subscription group in AppStoreConnect. You are specifically asked to order them in descending order, the first being the subscription with the most benefits, down to the subscription with the least benefits. They are easy enough to adjust if you need.
By doing this, you can be guarenteed that if a user upgrades from subscriptionA to subscriptionB, then subscriptionB will be listed in the latest receipt information when receiving a server notification or validating a receipt. If you get the order incorrect, you will have users trying to upgrade, but being stuck on the subscription with less benefits because your server will not see any higher subscription.