I think it's natural for someone who knows they are dying to experience grief and fear, even if it is juxtaposed with peace and acceptance. Important caveats here are people who are unable to comprehend death, possibly including young children, those with severe intellectual disabilities, and those whose terminal illnesses interfere so severely with their ability to understand what's happening. Another exception to this would be those who die suddenly, i.e. in a car crash or due to a heart attack, and don't have time to process that they're dying. Just my speculation on the matter, though.