Some of them, it seems like it could be to show the sauce more clearly:
https://www.mcdonalds.co.jp/en/products/4530/
But others, it's just inexplicable:
https://www.mcdonalds.co.jp/en/products/1010/
Burger King isn't doing this though (close the two popups to see the menu):
https://www.burgerking.co.jp/menu
Is it some kind of trendy style? It does feel kinda... cute.