I'd love to have something solid to link and show you, but this is isn't something I've seen written about hugely, and just something I've noticed so I'm just going to ramble here for a bit about what I've seen.
So first of all, you'll notice that your brand search (Barbieri Orthodontics) doesn't pull up a normal local 3-pack, it pulls up a branded 3-pack. You can read more about the branded results
here, but the main differences are the letters (A B C) instead of numbers, and no review stars.
The bigger difference though, is that Google displaying hat branded 3-pack means that it knows you're asking about a multi-location brand.
The next important piece of the puzzle, there's actually two kinds of knowledge panel that can show up. The local one, and the brand one. A local one is generally really easy to get showing, and tends to pull heavily from the GMB profile for content. The brand knowledge panel is a lot harder to get, and pulls from places like wikipedia.
So, here's what I've noticed. I can't think of a single time I've seen the branded 3-pack showing with a local knowledge panel showing to the right, it's always been a brand knowledge panel.
On the other hand, I've sometimes seen a multi-location brand search pull up a knowledge panel for the closest location to where I'm doing the search from, but then not showing a map at all. That seems to be kind of rare though, I haven't noticed that happening for many businesses. (There's a local burrito chain I like, an incognito search from my house shows the closest one, vs brand + city shows a branded 3-pack instead). You said there was a knowledge panel showing earlier: did it also show the map, or just the knowledge panel for the closest location? Was the knowledge panel a local knowledge panel with review stars and a map, or was it a brand knowledge panel (really unlikely given your client, but just thought I'd ask).
I'd love to hear others weigh in on this, but to my view, it seems like your new client previously having a knowledge panel was the anomaly, not the current situation of not having one. One thing that could cause that that comes to mind... if one location was showing very well and the other locations were buried for some reason, it makes sense that Google would show the knowledge panel, and then once the other 2 locations started getting optimized and showing better, I could see that killing the knowledge panel. After all, which location would Google show a knowledge panel for when there's multiple locations?
