around the world, thousands of people may experience them

How a Rare Disorder Makes People See Monsters
...
prosopometamorphopsia, or PMO
...
One of the first visual depictions of PMO dates to 1965, when an
artist, who had a tumor removed from the left side of his brain, saw
distortions on the right half of people's faces. TNP, as the patient
was called in the case report, drew a smiling nurse in a white cap; a
pink vortex swirled where the nurse's right eye should have been. When
TNP looked at a doctor's face, he reported that "the eye became a
ghastly staring hole, cheekbone a cavity; he had teeth on the upper
lip, often had two ears" on the right side.
...
...
Distorted perceptions are not the same as hallucinations, Blom told
me. If you saw an elephant appear in your home office, you would be
hallucinating. But, if you looked up and perceived an elephant in an
elephantine cloud, that's more like a distortion. "There's a
cloud--it's actually there," he said. He views his PMO patients as
very different from psychiatric patients with schizophrenia, who hear
voices or see things that don't exist. People with PMO aren't helped
by antipsychotics; they know that what they're seeing isn't right.
Blom suggested that PMO could fall under the umbrella of Alice in
Wonderland syndrome, a collection of neurological symptoms that can be
provoked by migraines, epilepsy, viral infections, or tumors, and
which distort a person's perception of their own body and the world
around them.

https://www.newyorker.com/news/annals-of-inquiry/how-a-rare-disorder-makes-people-see-monsters

with schizophrenia, who hear voices or see things that don't exist.

Don't exist applies to the observer's viewpoint, not the patient.
Their experiences can't be explained by today's scientists.  I would
not say those "things that don't exist."  That's speculation.