Apologies if this has been mentioned before, but why would an AI have a navel?
It's probably more of a navel-like dent without a lot of detail, and at the risk of drastically overthinking this, it's probably there because AI myomer muscles (at least in high-end chassis like Bubbles') seem to closely mimic human muscles. Bubbles has biceps, triceps, pecs, etc. so it would make sense that her abdomen follows the same muscle pattern as human abdomens, where the muscles sort of flow around the navel. It's also sort of a "landmark" and an AI with a navel would look a little more human-ish while wearing human clothes.
Similarly, AI feet don't need separated toes or a lot of details that would drag them into the uncanny valley, but they're close enough in general shape and function that most of them wear human shoes.
When you think about it, designing androids would be full of aesthetic and practical decisions and tradeoffs like this. You have to stay far enough away to avoid the uncanny valley, but at the same time you need to keep fairly close to a lot of human features we mostly never think about to enable good function and to give needed visual landmarks.
It varies, of course -- May's cheap-ass government chassis is a lot more abstract, and doesn't have a navel:
https://questionablecontent.net/view.php?comic=3175Then again, this is still a cartoon. The Benevolent Creator draws wonderfully expressive hands, but no one, human or AI, seems to have fingernails. It would probably take twice as long to draw each frame...