It would break mine. If you say it won’t break yours, that’s your call. This is a gray area.You are getting sugar from the actual lemon and chemical fake sugar from the stevia. So you are dirtying up your fast.
The main goal here is to learn how to not eat. To learn that hunger passes. So if this helps you early on to learn that discipline, it’s good for you.
Later, if you get to a point where you want to make some deletions or fast cleaner, this is something you can/should eliminate.