Odd compilation error with explicit operator

#1 
module internal MyModule

type MyChar =
    | Char of char

    static member op_Explicit(myChar): char =
        match myChar with
        | Char c -> c

module MyCharTest =
    [<EntryPoint>]
    let main _ =
        let mc = Char 'F'
        printfn "%A" mc
        let c = char mc // error FS0001: The type 'MyChar' does not support a conversion to the type 'char' 
        printfn "%c" c
        0

If I remove internal keyword from the first line, it compiles and runs well. Why can’t I call the explict operator defined in the type contained in the internal module?

For comparison, the following equivalent C# program compiles without an error:

using System;

internal class MyClass {
    class MyChar {
        char _mc;

        public MyChar(char c) => _mc = c;

        public static explicit operator char(MyChar myChar) => myChar._mc;
    }

    class MyCharTest {
        static void Main() {
            var mc = new MyChar('C');
            char c = (char)mc;
            Console.WriteLine(c);
        }
    }
}