Doesn't matter what the reason is DL. If the mum leaves it then my view is I must respect the mums decision. There are always what ifs. You and she have a very good hatch. Let it go.You're right. I don't like it. But I do respect it.
What if it's just a late hatcher because it was on the outside of the clutch?
I thought the same thing about her leaving it, that there might be something wrong with it. But what if it hatches tonight and I put it under her and she does accept it?
If it dies or she kills it after that, then at least I know I did everything I could.
You might spend hours or days trying to save it only to find it has some problem that isn't apparent at this stage. I've been there and done that. I trust my hens. They know a lot more about the business than I do.