She seemingly only had sparse memories up till that point of what Marika's life was like before, but not all the details. Like for example, the previous chapter brings up that she couldn't remember the time when Marika went to the amusement park with her parents until she happened to find the old picture of it that Marika hid away.
Farther back than that, if you go back all the way to Chapter 25 she mentions her trouble remembering things prior to the divorce.
Prior to that point it was just arbirtrary and not settled on by the mangaka. Go back farther to Ch 7 and no such thing with her classmates, or chapter 10 where she remembers prior to the divorce...
Your example is a good example of the issue. As you said she remembers the park randomly even prior to finding the photo and getting more details. Yet in the latest swap there wasn't any vague sharing.