If Bram Stoker's novel and Castlevania are linked -- and according to 1990's era Konami, they are, via Bloodlines (never mind the numerous plot holes this introduces) -- Dracula's fatal weakness to sunlight suddenly makes no sense. Dracula in the novel did plenty of frolicking around in daylight; he just couldn't shapeshift and had the physical strength of an ordinary man with his sort of build while he did so. But! He still had his control over creatures of the night and a sort of supernatural charm that could make people do what he wanted.
So Portrait's ending where Dracula dies in the sunlight (it's the only canon one I could think of) makes no sense. Dracula could have just swished his cape and gone "MUAHAHAHA! See you next time, Hero Kids!" and raced out of his tower into the sunlight. He couldn't do a badass transformation into a bat or a wolf, but he could have escaped unless something changed in that resurrection.
What I'm saying is that the sun should logically limit Dracula's abilities, but it shouldn't kill him.
So with that in mind, my personal headcanon for Portrait's ending is that the sunlight jammed Dracula's ability to heal his injuries, making the wounds that Jonathan and Charlotte inflicted on him into fatal ones. Being a vampire, he crumbles into dust after he succumbs to those wounds and dies.
So the sunlight didn't kill him -- the player characters did. The sunlight just enabled it at the eleventh hour.