Yes I'm aware of some past discussion, and the choice may have been deliberate, but that doesn't mean it's not fundamentally broken.
By fundamentally broken I mean that the subtyping relation is transitive, yet you immediately reach a contradiction here:
typealias S = (a: Int, b: Int)
typealias T = (c: Int, d: Int)
typealias U = (Int, Int)
var s: S = (a: 1, b: 2)
var t: T = (c: 1, d: 2)
var u: U = (1, 2)
// read X <: Y as X is a subtype of Y
u = s // S <: U? yes
u = t // T <: U? yes
t = s // S <: T? no
s = t // T <: S? no
s = u // U <: S? yes; T <: U and U <: S implies T <: S
// ... but we just said that T is not in fact a subtype of S...
There is no need to allow arbitrary addition of labels in order to allow reordering. You can have a well defined subtyping relation that says all shuffles of labeled records are subtypes of one another, i.e. fundamentally the same type.