Your blood sugar goes up with intense exercise (glucose created by your liver). Ketones and body fat take a while to create and metabolize and with intense exercise it needs quicker energy. Muscles will create lactate which will be recycled into glucose by the liver.
Intense exercise really isn’t helpful for fat loss for this reason.
Fake sugar is not good for you and can raise your blood sugar levels. (They can also drop rapidly after raising) Also exercise for some reason can also temporarily raise blood sugar levels right after.