Let's start with an explanation of how the iPhone X (and really, virtually any cell phone and less expensive cameras) take a flash photo.
What is Slow Sync Flash?
The iPhone X's rear camera comes with a quad-LED True Tone Flash: It evens out your photo so that your subject isn't washed out. In addition, Apple has packed in a new software feature — Slow Sync Flash — that combines the True Tone flash with your iPhone's image signal processor (ISP) to slow down the shutter speed while firing the flash.
Portrait mode greatly benefits from Slow Sync Flash.
Why is this important? If you're shooting in low light, three things are important to expose your image properly: A large aperture (the size of the opening that hits your camera's sensor), ISO (your iPhone's light sensitivity), and a slower shutter. A slow shutter lets more light into your camera when you press the capture button, thereby making your image brighter.
Because the iPhone's lens apertures are fixed (f/2.4 for the telephoto lens on the iPhone X, and f/1.8 for the wide-angle lens), your iPhone automatically tweaks ISO and shutter speed whenever you shoot an image with the Camera app to try and get you the best image possible. (If you use a third-party manual camera app, you'll be able to adjust these values manually.)
In older iPhones, when you turned on the Flash setting, your iPhone would match the shutter speed with the brightness of the flash; as a result, the subject in front of the flash would be evenly exposed, but anything not covered by the flash would be overly dark.
Here's the problem, which has been the bane of flash photography for decades. Yes, all the way back to the beginning of film cameras. The problem is, and always has been…
Slow shutter speeds combined with a flash is highly likely to produce red eye. Particularly in low light situations where the subject's pupils are already dilated.
The most commonly used flash sync for less expensive cameras is 1/60th of a second. This includes your iPhone X, Galaxy, Pixel, Android, and any other cell phone. You bought an advanced phone, not a professional camera. That tiny lens and minuscule flash is not professional equipment. It's equivalent to any $50 digital camera you can buy at Target. That slow shutter speed is asking for flash induced red eye.
Not to mention, all of the images any cell phone saves are highly compressed JPEGs. Take a photo of the same subject with a real, high quality camera that allows you to save your images in a non-lossy format, such as TIFF or RAW, and your phone (even in good light), then compare the two. You'll see massive detail loss in any cell phone, or cheap digital camera shot. The goal is to save space on your device so you can take lots of photos. Problem is, too much JPEG compression also destroys your photos. You can tell what they are, of course, but they'll never be as sharp and detailed as they could be.
The minimum shutter speed to avoid red eye is 1/125th of a second. 1/250th is preferred. Even on my Nikon D800, the default is the same stupid 1/60 of a second if I leave it on Program (automatic) mode. So I put it on shutter priority instead and set the camera to a 1/250th shutter speed. The camera and mounted SB-600 flash talk to each other. So, the flash knows I've sped up the shutter, and compensates with a brighter flash to produce enough light for the conditions.
You'd think this wouldn't make a difference. The speed of light should outrun any shutter speed and produce red eye regardless. I've never looked into the physics behind it, but it works. I can point the flash right at people in low room light and not get red eye in any shot.
In short, you're expecting a $50 camera to perform like a $2,500 one.