The problem arises when you have an app that's been free for a while, becomes paid... then is unlisted and relisted in the store. Most apps that become paid don't go through unlisting/relisting so they don't experience this bug. But there are a few like Toilet People, My Conference Calls, now of course Shelter CSG, Prognoza and many more.
I am a very active user of apps
so I've run into this a lot. Discussed it with Microsoft support a lot too. It's been going for years. I don't really notice it often as most apps are no keepers for me. But an app like Shelter.... That sticks out, it's probably the only game I play every few months. Maybe it's a sign I should pay anyway...
Users can still buy the app on Windowsphone.com. I've tried that with Shelter and it works.
Perhaps avoiding relisting an app would work. But once the problem pops up, it's there forever.
On a sidenote, please add an option for the old music, it was absolutely AWESOME!