There's a chance that the drawing was deleted- you shouldn't be concerned, it's happend to me before, and it usually happens when an artwork is deleted-
Also I saw an drawing you made called "Finish this drawing" and upon clicking on it it said "page not found" and after refreshing grid Paint it turns out that drawing was never there. Should I be concerned???
Yeah I used to do the corner sun but now I always put my sun relative to where I want the highlights and shadows to e or sometimes just what time of day I want it to be