From Kathāvatthu of the Abhidhamma @Yasoj has mentioned in another post this following untraceable sutta that is in it.
PTS p.169 (sl.221)
“Under the Exalted One Kassapa, Ānanda, I lived the higher life for supreme enlightenment in the future”
I have come to the conclusion that probably sarvastivada have the correct tradition and that once Theravada had the same tradition. There came a disagreement probably after the two split up as seen in point of controversy. Or as I said the correct suttas or tradition got lost and even didn’t get transmitted.
It is enough with this qoute to assume that the Theravada lost the tradition and had to make another story of when he started the Bodhisattva carreer officially.
But as noticed in this qoute, that it might be really ancient because Buddha does not mention that he vowed to become a Buddha but it’s translated just as supreme enlightenment in the future. Which can be like vow but it’s more nice said as expected from Buddha.
So I believe that’s the reason we still have the sutta mentioning as Ānanda said the Bodhisatta staying for his WHOLE life-span in Tusita Heaven. It early emphasize against the early Jatakas believe that after he went in Tusita heaven his career as Bodhisatta started officially. But in Jatakas before he lived under Buddha Kassapa he is not actually yet a Bodhisatta.
Probably later traditions started to create the believe in that he took many vows under many Buddha until he made this last vow. But as is evidently from Suttas Buddha Kassapa was the one Buddha lived under. And Agamas beautifully portrays a little bit more information.
Thank you. 