Everything about Rena's change in behavior between the lake and mall appearances, plus Yotsuba's lack of surprise this chapter, plus the previous suspicions she had about Itsuki, makes sense if they started out working together for the lake rena, but then Itsuki did the subsequent appearances on her own.
They collaborated on Lake Rena to get Fuutarou to move on, per Yotsuba's wishes.
After a while, Itsuki began to think it wasn't right - she could tell that Yotsuba loved Fuutarou, and she also knows that Yotsuba changed Fuutarou's life, and was probably his first love, since he held on to that photo for 5 years. As her sister and his friend, and knowing both of their feelings, she truly felt they should be together.
Thus, Itsuki did the Mall Rena thing (possibly with Raiha's help to get Yotsuba to the food court), and she was sad when it somewhat backfired and Fuutarou had seemingly moved on.
She still wants the two of them to get together, but she doesn't know what to do, so she decided to just meet with him as Rena again in Kyoto, get him to take another picture, etc. Now she wants to come clean about everything with Yotsuba together, but Yotsuba is holding herself back.
I think that the next arc or arcs will have her be in the awkward situation of knowing both Fuutarou's and Yotsuba's stories/secrets, believing them to deserve each other, yet also wrestling with feelings of her own, and not knowing how to deal with this dilemma.
The fallout could be used as a springboard for either a Yotsuba or Itsuki endgame... especially if it turns out that Itsuki has been hiding her feelings in a similar fashion to Yotsuba all this time.
They'd essentially both have the same reasoning for doing the bell kiss, as letting go of their feelings. Itsuki even more so since she'd be doing it as her normal appearance.
