Many recipes like stuffed peppers call for baking already fully cooked rice for upwards of an hour at ~375 F. I’m surprised the grains come out intact and texturally not that different than before they were baked. Why does the rice come out not tasting overly mushy or over cooked? 
Rice does taste a little different after baking, and its texture is also a little different, but the reason it is not mushy or overcooked is that the recipe controls the moisture levels. Totally uncovered it would dry out completely in the oven, and with too much water it would become mushy, but inside something like a stuffed vegetable or sealed pot the extra cooking time doesn't have too much of an effect. 