Most likely because Ella in the book wasn't stoopid and genuinely liked Kahir so why would said woman who knew about the rumors still ally with someone Kahir hated literally stoopid decision.
ella went back in time not transmigrated, in her first timeline she lied and acted as though she loved him but it fell apart i think after they had a child and she was getting jealous and she started acting as a tyrant in her own right. i think now she has a twisted view of setting things right with major and becoming a good wife to him but as you can see it’s super twisted
Ooh so she's an idiot before and after she got back in time ¯_(ツ)_/¯
I think I saw some spoilers that this Ella is transmigrated too. I'm wondering if the Ella from the book was also introduced to Kahir by empress dowager. If so, Kahir couldn't have trusted the book Ella so much. Could it be that the current Ella is so stupid that she allied with empress dowager to get close to him?