In short answer, we don't have almost any near-contemporary accounts on Ubbe/ Ubba before around the turn of the millennium, as I summarized before in: Vikings and Ragna Lothbrok and his 5 sons, how accurate is the History Channel show?, in contrast to other alleged Ragnarssons. Main source on him was the hagiographic literature mainly produced in Late Anglo-Saxon England (and further later), and, if we suppose that he was based mostly on a real historical figure (even that I regard as far from certain), it is likely that their authors didn't have access or pay very little attention to the oral tradition among the Danes/ Norse people, together with his possible moniker, that would be incorporated into the later traditions of Ragnar and his alleged sons in medieval Iceland.

As for how to consider the tradition of Ragnar and Ragnarssons primarily in Old Norse literature, it might be useful to check How can Ragnar Lodbroks children be considered real historical figures, but Ragnar himself is still considered to be mostly fictitious?, answered, among others, by /u/sagathain.