I’m not qualified to define “breaking a fast,” but I have heard the body responds to artificial sweeteners with a small rise in insulin. During a fast, you become insulin sensitive, so that spike could increase.
If nothing else, I think this will be detrimental to your sleep and render the melatonin not worth it. As to further implications on your fast, maybe someone else can chime in. I’m on a fast now and I’m going longer and feeling better than I have on my previous ones and I attribute it to not sweetening my electrolyte water for the first time. I don’t think it’s worth the insulin spikes.