Different behaviour on F# module initialization

I have the following F# program:

open MyModule

printfn "%d" test

With MyModule being:

module MyModule

printfn "foo"

let test = 
  printfn "bar"
  42

This produces the following output:

foo
bar
42

When I change MyModule to:

module MyModule

printfn "foo"

let test = 
  // printfn "bar" <-- note the comment!
  42

... the result is:

42

Why doesn't "foo" get printed anymore?