Again, might be possible I've got this wrong. But I'm pretty sure 'The Doctor Falls' was always meant to be Capaldi's last episode, but Chris Chibnall didn't want his first episode to be a Christmas Special. Moffat was a bit concerned that if they didn't have a Christmas special between TDF and TWWFTE that they would lose the Christmas slot, so he wrote Twice Upon a Time to serve as his & Capaldi's final episode just so that Doctor Who didn't lose it's Christmas Day episode slot.

Then, the next year come Christmas time apparently Chris Chibnall "Doesn't have any ideas for a Christmas episode", and so just casually throws away the slot which Moffat went out of his way to secure.

I'll admit, I am most certainly biased - I prefer Moffat as a writer and, frankly, as a person. So I may painting Chibnall worse than he actually is. But yeah, from my perspective, all of these decisions are just disrespectful. Of the fans and of the Moffat era.