I doubt it's an oversight, but it's difficult to come up with a coherent theory that explains her continuing reversals. It's even harder to come up with a theory without extremely unpleasant implications one way or another.
As I've said before, we really need answers to basic questions about Bella. I love a good mystery as much as the next man, but this one is going to go extremely rancid if it's left buttoned up much longer.
I'm still of the opinion that, at some point, the plot will revolve around "Isabella's awful, terrible, no good day." Not sure what it is going to entail, but if she and MC fuck and her husband shows up, I wouldn't be even slightly surprised by her at least attempting suicide. Obviously, it doesn't have to be that, but she's got some trauma, and that shit will fuck you up.
But maybe she goes on an ax murder spree instead. In that case, well, Josy, Maya and Sage all better look the fuck out!!!!! Bella is either going to be jealous for herself, or on Jill's behalf. Pretty sure Jill is safe, but who knows, maybe Bella's first victim was Jill's sister!