Jim said Harry didn't know about Maggie. That suggests that knowing about Maggie would change his emotional stance enough to break the protection.
Now that is an interesting point. Harry definitely feels way different about Susan after he learns about Maggie, so I think you might be onto something. And indeed, in that case Mab's explanation could, potentially (though I'm still not 100% on board) maybe work.
I thought it was so Harry though, when he asked Mab if she was saying that death defeats love, because it's obviously a Princess Bride reference. I think, of all the excuses, like Harry here, I don't want to believe that death changes the protection, though. That's so depressing. I'd say Harry's self-loathing, his (IMO) self-centerness in his grief, his guilt, and how belief tends to work in the Dresden universe would be slightly more convincing a reason for why the True Love™ protection goes away. Though still a cheap excuse. If that were the case though, if Harry starts to feel better about it, if he embraces Murphy's memory, would the protection return? Probably not.
I will say this, too, that I found a bit hard to digest in this book: I don't have a problem with Harry grieving, I'll start with that, it was necessary, it is raw and it is painful. I wished we had seen a little less of "Karrin is dead and oh how much this hurts me" and more "Karrin is dead and that's unfair because Karrin was x, y and z" I mean, he tells us more times how wonderful Michael is in any of the books than he thinks about Murphy in this one, when it'd be the appropriate book to do it. There's a lot of things that you feel when you lose someone, yes, guilt, anger, denial, but also longing, and the memories of the best about them replay in your head making you miss them more, want them more. I felt there wasn't enough longing or enough memories. And I still can't believe we didn't learn when her birthday was in a full year of story.