Your water is at the hard end of “soft,” but not hard enough to be called even moderately hard.
It also makes for delicious bread.
Very hard water can lead to more foaming; that’s why laundry detergents generally include water softeners. But that’s not the case, here.
Also, it’s definitely not the isomaltulose. As you noted, it’s not sweet - isomaltulose is half as sweet as sugar, but it’s still sweet. Also, there’s less than 44 g of isomaltulose in a package - isomaltulose is about 20% soluble in chilled water. 20% soluble in 2 liters of water means 20% by mass into 2000 grams of water, or 400 grams of isomaltulose should dissolve and stay dissovled.
Fats, on the other hand, are harder to blend into water without aggressive mechanical means. If our store-bought milk were not homogenized first, all the cream and milkfat would float to the top, too.
You might think that a powdered fat is already in tiny sizes and be homogeneous to start with, but maybe not. I don’t know. I’m not sure how small the micro-droplets need to be before the liquid behaves like it’s homogenized.
But I do know that emulsifiers help prevent fat micro-droplets from coming back together, clumping and floating to the top. Many soluble fibers act as emulsifiers, but Soylent reduced the level of fiber, so this may be allowing the fat to separate more easily. Combine that with the fact that there’s now more fat than before, and we may be looking at the answer.