Weird typed class typechecking behaviors

What version of Racket are you using?

Racket 6.7

What program did you run?

#lang typed/racket/gui

(define-type Separator separator)
(define-type StyleSheet stylesheet)
(define-type Subject subject)

(define-type Contract-Violation-Separator%
  (Class #:implements Canvas%
         (init-field [parent (Instance Area-Container<%>)]
                     [css StyleSheet]
                     [parents (Listof Subject)]
                     [inherits (Option HashTableTop)])))

(define-type Type-Blind-Separator%
  (Class #:implements Canvas%
         (init-field [parent (Instance Area-Container<%>)]
                     [css StyleSheet])))

(struct stylesheet () #:transparent)
(struct subject () #:transparent)
(struct separator ([width : Positive-Byte] [color : String]) #:transparent)

(define separator-filter : (-> Separator) (lambda [] (separator 4 "Snow")))
(define css-cascade : (-> (Listof StyleSheet) (Listof Subject) (-> Separator) (Option HashTableTop) (Values Separator HashTableTop))
  (lambda [stylesheets subjects filter inherits]
    (values (filter) (or inherits (make-hasheq)))))

(define hline% : Contract-Violation-Separator%
  (class canvas%
    (init-field parent css parents inherits)
    
    (: hs Separator)
    (define-values (hs _) (css-cascade (list css) (cons (subject) parents) separator-filter inherits))

    (super-new [parent parent]
               [style null] [paint-callback void] [label #false] [gl-config #false] [enabled #true]
               [vert-margin 0] [horiz-margin 0] [min-width #false] [stretchable-width #true]
               [min-height (separator-width hs)] [stretchable-height #false])
   
(send this set-canvas-background (make-object color% (separator-color hs)))))

The hline% should have been the main issue, however in this example it works well. But
I met it before in a different situation in real world code

static-contracts/instantiate.rkt. car: contract violation.

I still not sure how to reproduce it consistently. During the process of trying to reproduce it, I found another issue fantastically:

(define vline% : Type-Blind-Separator%
  (class canvas%
    (init-field parent css)

    (: hs Separator)
    (define-values (hs _) (css-cascade (list css) (list (subject)) separator-filter #false))
   
    (super-new [parent parent]
               [style null] [paint-callback void] [label #false] [gl-config #false] [enabled #true]
               [vert-margin 0] [horiz-margin 0] [min-width #false] [stretchable-width #true]
               [min-height (separator-width hs)] [stretchable-height #false])
  
 (send this set-canvas-background (make-object color% (separator-color hs)))))

Type Checker: missing type for identifier;
consider adding a type annotation with `:'
identifier: self6196653 in: css

There also are other strange behaviors if the code exists in class body. Some are not reasonable; Some are easy to work around, some are heavily affected by context.