I think i found the problem. The subscription status was not checked every time, it was using old cached data. So even if you canceled it on sandbox, you still wouldn’t have seen the upgrade screen the reviewer was talking about. Indirectly the rejection kind of found a bug. With the current implementation a user could cancel the subscription and continue to use the app indefinitely.

@App Review Thanks, I don't know if that's necessary, if someone from the review team has a couple of spare minutes that would be nice, but like I said in the reply above, I think i found the issue. Hopefully that is what the reviewer meant when they said they can't get to the in app purchases screen and it's not something else. I have resubmitted the app and we'll see if the issue is fixed once they reivew it again.