Minimal example of local-expand not turning lambda into #%plain-lambda in Racket

mystery-lambda.rkt

#lang racket

(define-syntax (show stx)
  (syntax-case stx ()
    [(_ x)
     (let ([fully (local-expand #'x 'expression #f)])
       (displayln (syntax->datum fully))
       #'x)]))

; expect to see (#%plain-lambda (x) x), but get (lambda (x) x) instead
; docs for fully-expanded Racket: https://docs.racket-lang.org/reference/syntax-model.html#(part._fully-expanded)
(show (lambda (x) x))