Information on inserting shadow characters into UIImage

I'm not sure why, but I was able to see that if the conditions listed in the comment include full-width characters (please include these conditions in the body of the question as well as in the comments) no longer draw shadows (on Playground in Xcode 8.3.3).

(By the way, in Xcode9GM seed, just matching your code to the correction in Swift4 has also shadowed strings that contain Japanese.)

Of course, there is no description in the document that certain attributes will be disabled under certain conditions, so it would be better to simply report Apple as a bug.

Now, What is the solution to the above problem? seems to be the subject of your question, so I'll post a code that says it worked in my environment.

let font=UIFont.boldSystemFont(ofSize:20)

let imageWidth = image.size.width
let imageHeight=image.size.height

let rect = CGRect (x:0, y:0, width: imageWidth, height: imageHeight)

UIGraphicsBeginImageContextWithOptions (image.size, false, 0)

image.draw(in:rect)

let textRect = CGRect (x:0, y:0, width: imageWidth, height: imageHeight)
let textStyle=NSMutableParagramStyle.default.mutableCopy() as!NSMutableParagramStyle

let shadow=NSShadow()
show.shadowColor=UIColor.red
shadow.shadowOffset = CGSize (width:5, height:3)
shadow.shadowBlurRadius=3

let textFontAttributes = [
    NSFontAttributeName:font,
    NSForegroundColorAttributeName:UIColor.black,
    NSParographStyleAttributeName:textStyle,
    NSShadowAttributeName:shadow
]

let attrText=NSAttributedString (string:text, attributes:textFontAttributes)
attrText.draw (in:textRect)

let newImage=UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext()

The difference from your code is that instead of calling the draw(in:withAttributes:) method of String, you first create NSAttributedString and then call that draw(in:) method.

I haven't been able to investigate whether it's a build or a running environment issue, but what will happen in that environment?Try it.