I don't think adding stuff like fruits will benefit the soap, some people may use it for label appeal. And most often than not such stuff discolors.
Maybe you are cutting when the loaf is too hard and the wire doesn't glide smoothly? How wide is it (the wire)?
Or maybe the peel is not finely ground enough and it gets in the way when you cut, resulting in those 'waves'?
Or you can lower the SF and let it add to it naturally, without bothering with math. I'm only saying this because some calculators do show how much NaOH gets consumed by citric acid (for example), but don't have info about the acidic content in juices, (which can vary greatly), making it hard to calculate manually.
For example, using lime or lemon juice (both high in citric acid) as a full water replacement in a recipe with 0% SF and 40% lye concentration will result in soap with SF in the range of 3-6% in most cases (of course, depending on your particular recipe). It would be nice to know exactly how much citric acid a bottle of the juice we bought or squeezed contains to be more precise with the recipe, but the average soaper can't determine that, unfortunately - it's a guessing game.
How about splitting the water between the powder and the NaOH? Dissolving both in separate containers, then mixing the two?
Maybe fruit powder is just prone to that no matter what you do. Or maybe it depends on the fruit. I think ALJ powder doesn't act that way, from what I've heard