My thing about BR!Tubbo is that he seems to know Theseus but know nothing about him at the same time. Sure he knows that Theseus doesn’t like floral tea because it makes him nauseous, but he doesn’t seem to understand that Theseus is rather blunt about things that get in his way or make his life difficult. He dislikes Ranboo, because he’s in the Emperor’s favor and is being shoved onto him, and that makes his life difficult so he’s obvious in his dislike and damn near hatred of Ranboo. Tubbo also spilled Theseus’ secrets, he should’ve taken those secrets, regardless of their nature, to the grave, especially that one about the letters, because he had no right to share it and it wasn’t hurting anyone.
It's nice you actually put au in the title because people seriously think Tubbo doesn't care about Theseus in the original br because he left him, in Chapter 19 he says he left because Theseus always chased him away and that he was practically useless to Theseus after he became crowned prince so he thought Theseus didn't want him around anymore so he left for him to get a better aide not knowing that Theseus still wanted him around, it doesn't help that when Theseus found out Tubbo was leaving him all he ever jabbered about was that Tubbo was gonna be a commoner's aide and if he had wanted Tubbo around and didn't want him to leave he would've said so which is why Tubbo made his final decision