If you’re fasting for weight loss and keeping within your calorie targets, it won’t set you back other than possibly training your body to start expecting night time snacking on weekends, which you then have to work on overcoming in the following days when you shorten your eating windows again. It might turn into a kind of a cycle that psychologically might make you frustrated in your IF journey. Though some variations in your window isn’t bad in and of itself (because some folks don’t want to become too accustomed to a single window, either.)
Night time snacking may also negatively impact your sleep, too, depending on how close to bedtime you’re doing it. So keep that in mind, too.
But, if you’re doing IF for the additional benefits like insulin sensitivity and increased autophagy, then yes, having a longer eating window where you’re likely to eat more frequently (even if within your calorie target) will set you back on those goals a bit, because when you eat, you increase your insulin, and interrupt autophagy altogether. But, you may decide the trade off is fine if you’re strict about your windows the rest of the week.
If your goal is weight loss and you’re happy with the results you’re seeing, then I wouldn’t sweat it unless or until the results stop.