Set the default ringtones (in Settings -> Sounds and feedback) to custom ringtones, then put different ringtones as the actions for a certain ambience (in Settings -> Ambiences). Once you switch to this ambience from your current one, your ringtones will change to the ones defined by the ambience. The problem is, this change in ringtones is not temporary, instead it actually changes the default ringtones you just defined in Settings. This means you lose all your default ringtones every time you change to an ambience that has custom ringtones, and the ambience ringtones become the defaults. Going back to your previous ambience has no effect, unless you had also put custom ringtones on that one, and then that ambience changes your default ringtones once again. To top this off, even if you remove the ringtone actions from Jolla's ambiences, they return after every update, and if you happen to select one of Jolla's ambiences before removing those custom ambience ringtone actions again, you'll have a lot of fun setting up all your ringtones yet again, every time.

Complicated explanation, but the problem is simple, which is precisely why it's so frustrating.