Disney's Frozen 2 has a twist revealing that Elsa is really the fifth spirit, and it's similar in terms of execution and problems to Doctor Who season 12's Timeless Child. Frozen 2, which is now streaming on Disney+, sees Elsa effectively die and be resurrected, but revealed to be the immortal fifth spirit, who has the power to unite the other four elemental spirits. In Doctor Who season 12, a similar twist occurred when it was revealed that the Doctor (currently played by Jodie Whittaker) wasn't actually the Time Lord she thought, but instead a being known as the Timeless Child. Rather than being a product of the Time Lords, the Doctor was the very beginning of them. The Timeless Child has unlimited regenerations, and that power was then harnessed by the would-be Time Lords after the child who'd eventually be known as the Doctor was first discovered.
While Frozen 2 and Doctor Who are not exactly similar in terms of story, both of these twists are similar in nature. Each changes something fundamental viewers know - or think they know - about the main character, and the established mythology alongside it. They're reveals that go back to the very origins of Elsa and the Doctor respectively, making both something ostensibly greater and more powerful than anyone realized (including themselves). Elsa isn't just a magical being, she's the most powerful of them; she's not simply connected to the elements, but unites and controls them. Likewise, the Doctor isn't the most noteworthy of the Time Lords, but pre-dates them and is responsible for their very creation.
Both Frozen 2's Elsa fifth spirit twist and Doctor Who's Timeless Child reveal are supposed to work in favour of the characters and their respective journeys, but instead end up undercutting them. They're twists that put mythology, lore, and big reveals ahead of actual character development, using plot twists rather than a strong story arc. There's little hint in Frozen that Elsa is the fifth spirit; instead, Frozen 2 chooses to build a new story for the character, interweaving her parents' deaths, and their own first meeting too, as part of a convoluted plot. Similarly, Doctor Who season 12 spent most of its time hiding the mystery of the Timeless Child, only to reveal it was the Doctor - which again, there were hints of, but it can't exactly be said it was setup.
The changes to mythology are more jarring for Doctor Who, which is dealing with over 50-years of established character work and storytelling (albeit with other retcons in there), than it is for Frozen 2, which, although based on Scandinavian mythology and the works of Hans Christian Andersen, is a relatively new story. Nonetheless, both seem to go by the idea that changes to lore and origin stories are enough to work as character development, rather than developing the characters in a more organic way.
That, ultimately, is why both Frozen 2's fifth spirit and Doctor Who's Timeless Child fail as plot twists: they're major changes to the backstories of their respective characters, seemingly huge retcons to canon, and yet they don't have all that much impact upon the characters themselves. Elsa, who went through a far more compelling journey in Frozen, is largely still the same person; a leader and a protector. Likewise, the Doctor is the same good, heroic person after the Timeless Child reveal as she was before it. The story serves the twists, rather than the twists serving the characters and narratives, and that's why they don't work.