But what if it isn't? What if, instead, they fixed it because they needed to use it? We have WoJ that Harry is first-rate when it comes to making magical artifacts, even compared to wizards with far more experience. Furthermore, Little Chicago took an enormous amount of time to put together, as well as a fair amount of resources. If someone needed to use something like it just once, as opposed to on a regular basis, it would make more sense to fix Harry's and just use that, rather than expend the time and effort to build their own.
Harry and Bob initially missed the flaw, though, so whoever fixed it had to have had foreknowledge that Harry was about to blow himself up and wanted to prevent that, and / or been even better with subtle artifact creation.
Using Little Chicago could fit as a motivation for Mab to fix it, though. She had means to get into the apartment so long as she behaves as a good guest, and opportunity in the form of abundant skill to notice and repair it. The main argument against her was always motive - that if she knew of the flaw, she wouldn't just benevolently fix it. Her usual nature would be to hit Harry up to trade another favour(s) for 'information which will soon save your life'. But if she found it convenient to use Harry's gear for some reason, fixing the flaw could be rationalized as her way of paying to use LC, while also preventing her asset from accidentally depreciating himself.
I doubt it's a coincidence that someone else needing to use the spy tool happened at the same time shit was going down for a Nemesis plot. It's a WAG, but if it was Mab was using it, the timeline fits for that being how she found out that Maeve was compromised. Possibly she couldn't be away from Arctis Tor longer than absolutely necessary and found it convenient to capitalize on Harry's groundwork. Or perhaps Maeve's surveillance countermeasures were focused on Winter magic, and a mortal spell was able to get around them.