Delete game data from iCloud/Game Center

I have a problem with the game Where's My Perry? by Disney Mobile

https://itunes.apple.com/gb/app/wheres-my-perry/id528805631?mt=8

I have completed the game several times and delete it off my phone when I am finished with it.

I want to play the game again from scratch. I have downloaded the game, but when I start it up, it shows that I have already finished the game (it says I have achieved 540 out of 540 gnomes):

I then go into the game settings, turn off iCloud sync, and tap the "reset game" button:

Note that the gnome count now says "0/540". I can then play the game.

BUT!

When I reload the game, it is back to "540/540" and the game is finished again.

I have turned this off in iCloud:

But it makes no difference:

Back to 540/540 again.

How do I permanently delete the data? Where is it getting the information from, that I have completed the game?

If I delete the app from the phone, it does't help. As soon as I re-download the file, it shows that the game is completed again. It is obviously getting the data from somewhere, but where? How do I find out where, and how do I delete it?

Disney Mobile have been worse than useless and say it must be a problem with my phone.

I am on the latest version of iOS 11.0.3 on iPhone 5S.

Any help would be really appreciated - thanks!