I think if the flavoring has artificial sweetener then it could raise blood insulin levels. However, it’s it just depends on what you are fasting for. If it’s simply weight loss, then it shouldn’t be an issue. If it’s for autophagy, then some studies have shown insulin increases has decreased the autophagy effectiveness.
I personally, drink LaCroix sometimes which have “natural flavors” and don’t really see any difference from a weight loss/fat loss perspective.