Quite simply because, he is a seer.
A seer is a gifted wizard or witch who has the ability to see
into the future with their Inner Eye. Seers predict prophecies, which
are then recorded and stored in the Hall of Prophecy in the Ministry
of Magic's Department of Mysteries. According to Minerva McGonagall,
true Seers are extremely rare, and she had doubt that ...
The scene/fear is explained near the end of the film.
When Leta is explaining her trip to the US with her baby brother constantly crying and screaming, she exchanges the crying baby with a silent sleeping baby. The ship then sinks and the original baby sinks to the depths of the ocean in a white sheet moving just as her Boggart experience.
You're actually taking the word of Grindelwald (who lies and charms his way to get things done) very seriously.
For all we know, Grindelwald could be lying. Here are a few reasons why I think GG is lying
GG did not know about Credence -
In 'Fantastic Beasts', GG only thought Credence was a squib. He only realised Credence was an obscurial and his power ...
It's just a change of heart. With time people get over a broken heart or at-least stop wishing to get it back.
That's what happens here; Dumbledore sees his lover in the mirror as he is still not over him and wish him back. But with time his wishes change and he sees his sister in the original film series.
With age and time, your priority and wishes change ...
We didn't see this bird in the Fantastic Beasts and Where to Find Them and in CoG we only see it in the house where Grimmson kills that servant who put him to adoption.
So, this is obvious that that bird only appeared at that house and Credence starts to take care of it. Later it is revealed that that bird is phoenix and came to him when he was in need.
We don't know for sure if he actually cast a spell.
We never saw him muttering any spell and for a wizard to cast a non-verbal spell, he'd have to be way way experienced (like McGonagall + level experienced). Also Credence doesn't know any spells yet. He has only raw power.
So here's what happened.
Just like Harry did in Harry Potter and Philosopher/...
Does the mirror show the past of the person?
That's one way of seeing it but in reality it show you your deepest desire
Harry: So, then it shows us what we want? Whatever we want?
Dumbledore: Yes...and no. It shows us nothing more or less than the deepest, most desperate desires of our hearts. Now you, who have never known your family, you see them ...
Since I answered this over on SFF I'll just copy and paste my answer here as it is the same question.
It is unclear as to who this McGonagall is, though it is almost certainly the Minerva we know from the main Harry Potter series.
The Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay by J. K. Rowling only states her to be a "young ...
I had my suspicion and this article proves it right.
In Chamber of Secrets Whomping Willow was in training grounds. But in Prizoner of Azkaban it was on the hill outside Hogwarts.
And from Pottermore,
In Prisoner of Azkaban, the Whomping Willow was moved from the training grounds to a hill outside Hogwarts, where it covered the entrance to the secret ...
FWIW you do see her portrait in the book as Flamel flicks through it.
FLAMEL flicks open another page. EULALIE HICKS, a young American professor at Ilvermorny, looks around, worried.
EULALIE What’s happening?
FLAMEL Exactly what he said would happen. Grindelwald rallies tonight at the cemetery, and there will be death!
EULALIE Then you gotta ...
Possibly, But Not Necessarily...
There are a few theories out there that could explain the BIG plot twist at the end of the movie that may pertain to THESE minor plot twists, because of whom the source of information is (Grindelwald), may not validate the information given.
Theories about BIG plot twist:
Percival conceived a child with another women while ...
As mentioned by Paulie_D in his comment or the respective answer from sister site it seems not to be a possibility for now but never say never as there suppose to be three more sequels planned to turn the tables, also to be remember Grindelwald is not that trustworthy narrator either.
Digital Spy also tried to dig into all the possibility to make it logical ...
The Mirror of Erised is one of those magical artifacts that seems to
have been created in a spirit of fun (whether innocent or malevolent
is a matter of opinion), because while it is much more revealing
than a normal mirror, it is interesting rather than useful. Only after
Professor Dumbledore makes key modifications to the mirror ...
This is because the Swooping Evil's venom was used in this particular Oblivation event, not the "Obliviate" spell. Swooping Evil venom only erases bad memories, Obliviate erases any memories the caster wants to, right up to almost completely wiping their memories like what happening to Lockhart in Chamber of Secrets.