The thing is that Meghan hasn't completely lost the baby weight. I think this style, with the belt, does work for her normally because it gives the illusion of shape on her short-waisted body. However, because she's still carrying a tiny bit of extra weight (and I am in no way slagging her off, she looks amazing), the belt is counter productive - she's still trying to create a bit of a silhouette but it's just not possible with her current figure.
Eg that dress she wore for one of her first engagements with the queen just after the wedding? That had a little belt and it totally gave her the suggestion of an waist.
I am actually a similar body shape to her (but on a much larger scale!) - short waisted, longish slim legs and tendency to gain weight around my middle. But there are times when a belt really does give me some shape and definition and other times (usually when I've put on a little extra weight or am a bit bloated etc) where it's counter productive.