Bug: can't pass duplicate `Number` literals to the same constructor

There's a strange bug with literals in Substance where diagrams break if you try to pass the same literal in multiple arguments of the same constructor. For example, consider this program that draws points with specified (x,y) coordinates:

-- example.domain
type Point
constructor Point( Number x, Number y )

-- example.style
canvas {
  width = 400
  height = 400
}

forall Point p; Number x,y
where p := Point(x,y) {
  shape C = Circle {
    center: (x,y)
    r: 2
    fillColor: #000
  }
}

--- example.substance
Point a := Point(0,1)
Point b := Point(10,11)
Point c := Point(20,21)

This example works fine, producing three dots along a diagonal line:

However, if we change the coordinates so that x=y for, say, the second point, we get a result like this (middle point disappears):

Point a := Point(0,1)
Point b := Point(10,10)
Point c := Point(20,21)

Likewise if we put point a at (0,0) or point c at (20,20), it does not get drawn.

Could it be that the Style compiler is treating two identical literals as non-distinct Number instances (hence the Style pattern doesn't match)?