Hi@leechiyong. Not sure what it was inside, seems like water stain but then I use wipes won't fade it.Weekly’s just my process for the new ones, then it’s every few months where I can also inspect them.
Moving closer to topic, I’m not sure if this would resolve the yellowing in the pictured bag, but inspection might have caught it earlier when the damage wasn't quite so bad.
The interior very much looks caused by something external. The exterior is hard to tell; I'd need more detailed photos.
Exterior, I really don't know what causing. It was sitting on the top shelves from the closet. Used few times and then leave it there, my mistakes buying the white one, afraid will gets jeans stain and now end up like this

Oh no, I hardly changing bags, stays in one till I found another haha but now using waist bag more (like a crossbody)