It is common
They are not in their underwear but in their swimwear
If you put the female sims in tankinis, they will look more "dressed" when this glitch occurs, the males have options of longer swimtrunks, though that will still leave them half naked. You can pick out the swim- and sleepwear for your sims in the wardrobe and when you send them to swim or sleep they will spin into the choices you have made
I really recommend picking out their swimwear so you do not end up with those light purple default swimwear, even if you do not choose the more covering options it is still A LOT prettier than those default swimwear they have
Since this is not game breaking, (meaning you can still play the game, and it is only visual, it does not effect what tasks you can or cannot assign them to nor does it hinder you from progressing in quests or events etc) this has a low priority on Firemonkeys to-do list but you are of course welcome to tell them that this bother you and that they should priotitise this glitch higher