Since

set(value) = ...

for custom setters on variable is also allowed, I'd say it's no problem.

Unit

is just a normal return type and value in Kotlin.

I know that it works, I'm asking whether the Kotlin Style Guide encourages to use expression body with a one-line Statement or not

it's indifferent on the matter. Common wisdom says, use them when they fit, don't when they don't, always specify return type in public API.

Our internal style guide suggests to always use body syntax for functions that return Unit

providing context https://www.reddit.com/r/Kotlin/comments/83hrpb/code_style_using_singleexpression_functions/

And thats why I'm wondering why people save 2 keystrokes (inside Intellij its one? additional keystrokes)

fun foo() = println("Hello World")

vs

fun foo()  { println("Hello world") }

because it's not in the official guideline and they have yet to stumble upon a problem caused by this.