But regeneration isn't a reset button, if anything in the Doctor's case the regeneration is influenced by the state of mind when regenerating.
That's not really true. Thanks to RTD inexplicably trying to throw a grenade under the bus with David Tennant's last story, in an effort to unnecessarily score cheap emotional beats, it's been clearly stated that regeneration is basically death for the current incarnation of a Time Lord, with the replacement incarnation being a totally different person.
That's not just implied by "I don't want to go," The Doctor explicitly states that's the case when he's at the café with Wilf.
Then we have Capaldi's 12th Doctor, which is not just a radical departure from Matt Smith and David Tennant, it undoes the Doctor's character development that had been underway since 2005. From the word go, Capaldi's Doctor is portrayed as darker, more ethically dubious than the two previous incarnations. That is what kicks off his entire character arc of undergoing an internal dilemma over whether or not he's a good person.
Ultimately, it doesn't matter what the excuse is for The Master's about face. The implication I took away from the opening two parter was that the about face stemmed from discovering the secret of the Timeless Child. But even if that's not reason enough, this isn't the first or the last time that Doctor Who will see continuity errors or character shifts related to a new creative crew. Otherwise Davros would still be a head in a jar.
