I think the problem isn't Tsuyu's personality changing. She's still the same shy but resolute girl from the pre-serialization. The problem is CHIE becoming more prominent and overwhelming Tsuyu's initiative. As an example, the way chapter 5 would have been done in the pre is for Tsuyu to get the idea to make a bracelet (she still does) but then go ahead with it on her own (like she did with the haircut in the pre) then realize that she didn't ask for big bro's opinion on colors but big bro accepts anyway and she's happy. The problem here comes when Chie takes the initiative and pushes Tsuyu to ask for colors. It's actually Chie's pushing that changes the trend of the series. She was neither this prominent nor this pushy in the pre.
In the cat chapter where there's no Chie the story proceeds the way it would have in the pre. Tsuyu is neither too shy nor irresolute. She tries to wile her way into big bro's attention and gets jealous when the cat instead gets it. She gets surprised from the headpat but reacts coyly like the old Tsuyu would have.
To be honest, though, I have always preferred Chie as a character, so I don't mind her becoming more prominent at the expense of Tsuyu, and I have a suspicion that a lot of the Japanese audience might have been leaning more towards Chie as well near the end of the pre because Chie was also starting to become a more prominent character there when it ended.