According to "an insider" who has been far more accurate than not with the leaks:
This is from the original ending to Ncuti's second season:
^ Allegedly shot in May 2024. The Doctor wins and everyone celebrates at a party where the Vlinx is the DJ (!). At least Ruby is there as well, some suggestions a bunch of UNIT people. The camera was going to pan up to reveal Susan watching from above and then into the end credits. Whilst baby Poppy would have gone back to her original reality timeline.
The original plan was for there to be a third Ncuti series, with Belinda as his assistant, which would have been about Susan, linking all the way back to Hartnell's Doctor, and trying to sort out the remaining timey-wimey leakages, Timeless Child, and "mavity" issues with the big bad being "The Boss".
However, as Disney wanted to do a wait and see before signing off the $$$ for a 3rd series taken up to the end of the agreed 26 episodes with the BBC, that meant filming could not begin (between August & December 2024 as originally planned, as Disney were more interested in filming The War Between the Land and the Sea spin off instead), so rather than leave Ncuti in limbo to miss out on potential acting roles, he decided to exit the role. Resulting in a hasty rewrite/edit, and in February 2025 filming his blue screen regeneration; with #13 cameo in the Tardis; and the Poppy is Belinda's baby retcon were all filmed.
So the final scene we all saw as a headline grabbing reveal is RTD's equivalent of a Screensaver....
So blame Disney!