Seen another report of this in the "Other" section of the forum. This seems to only effect iOS 10 with iOS 9 correctly returning a valid receipt location for both sandbox and release.
iOS 10 the URL seems to return a sandbox receipt which doesn't exist so it's causing issues enabling the subscription functionality for our users as we have no receipt to confirm purchase.
Would love any thoughts on this one.
Other thread on this issue: https://forums.developer.apple.com/message/183382#183382