"I only drink still water when I eat."
Translation:Je bois seulement de l'eau plate quand je mange.
It is, and the ambiguity exists in both the English and Duo's given translation. However, I suspect and wonder if a French native speaker agrees, that ne...que would more likely imply the latter meaning, ie the only thing I drink when I eat is still water. The ne...que construction always sounds to my ear like the archaic/poetic English use of 'but': 'I drink but still water when I eat', the meaning of which clearly pivots around 'when I eat'.