1st episode of new anime adaptation of light novel series My Stepmom's Daughter Is My Ex aka Mamahaha no Tsurego ga Motokano datta:
Yume Ayai and Mizuto Irido have recently become step-siblings. Before they were step-siblings they used to date. It seems they broke up at least 1 year before becoming step-siblings.
In the very 1st scene, we see that Yume calls Mizuto as Mizuko-kun and Mizuto calls Yume as Yume-san.
Later, they play Kaguya-sama-like games with each other to try to trick the other into not acting as normal siblings. They manage to trick each other at least once each. Now, also once each, when 1 of them gets tricked, they call each other by last name: Irido-kun, Ayai.
(Ignoring the issue of the presence/absence/choice of honorific...I'll just pretend for now it's the same as 'Himuro (no honorific)' and 'Yukimura-kun' in Science Fell...)
Question: What's the idea here? Is it like, Idk, 'simulating' when they 1st started to have a crush on each other and thus weren't so familiar with each other and so as part of the simulation they would call each other how they initially called each other then?