The mistake you’re making here is in thinking that Soylent is sweetened. It isn’t.
They do use a tiny bit of sucralose, but that’s used as a masking agent, not as a sweetener. The quantity of it is far, far too small to have a meaningful sweetening effect. The slight sweetness you taste is from other ingredients, like the maltodextrin and oat flour (neither of which is particularly sweet.)
Also, I make my DIY with stevia, instead of sucralose… and I still get the typical headache when going back on soylent after a break on which I eat a lot of more “typical” food. I’m all but certain it’s from the boost in potassium. Drinking extra water wont overcome the potassium effect, because it’s not a dehydration problem; potassium/sodium balance affects the ratio of pressures inside the cell versus outside the cell. Drinking extra water won’t change the ratio. (You do need to remember to drink water when eating Soylent to stay hydrated - drinking your food may lead you to think you’ve been drinking more than you have - but that doesn’t play into the “immediate headache” effect when you’re just starting to drink Soylent.)
Extra salt when starting soylent may reduce the change in ratio, but it still happens.
Sometime, when I’m on a break and eating “typically,” I need to remember to start supplementing with extra potassium for a day or two before going back on soylent, to see if that helps.