Not a bug really, just an inconsistency.

Code:
namespace Test
module Module1 =
let getFullName name surname = $"{name} {surname}"
let name = "Robert"
let lastName = "Smith"
let fullName = getFullName name lastName
module Module2 =
type FullName = FullName of name: string * surname: string
let name = "Robert"
let lastName = "Smith"
let fullName = FullName(name, lastName)