I just looked through again, paying better attention this time.
Momo's been getting regular bubbles since the
beginning of this arc. Both Charlotte and...the other one (did we get her name?) did, too. But Mr. Toaster didn't, and Pintsize hasn't changed either.
The last appearance before this arc of hers (that I found just clicking through, anyway) she
DID have a square.
You're not alone in just realizing it though. Probably should've gotten it when it was right there with Pintsize's squares, but I didn't. Not even sure I ever noticed the distinction before you pointed it out, to be honest. I've just noticed that each AnthroPC has their own particular not-human font. It's interesting that Momo
has retained that, as did the salesbots (though they had the same one). Maybe to point out that their voices don't sound entirely human?
So...maybe it's just the distinction between little robots and bigger, more human-y robots, where perhaps the voices sound CLOSER to a real person's, and it was just easier for Jeph to give 'em to Momo early?
Or perhaps I'm thinking into this too much and should get to bed.