2.8.1: Evaluation error when flattening ground instance

[informarte]

Consider this problem:

include "lex_less.mzn";

int: N = 10;
int: M = 9;

array [1..N] of var bool: x;
array [1..N] of var bool: y;

constraint x = [true, true, true, true, true, true, true, true, true, false];
constraint y = [true, true, true, true, true, true, true, true, true, true];

constraint lex_less(x, y);
constraint forall(i in 1..M)(x[i] = true);

solve satisfy;

output ["x = ", show(x), "\n", "y = ", show(y)]

MiniZinc 2.7.6 outputs the solution as expected:

x = [true, true, true, true, true, true, true, true, true, false]
y = [true, true, true, true, true, true, true, true, true, true]
----------

MiniZinc 2.8.1, however, fails with this error message:

Error: evaluation error: cannot evaluate expression 'X_INTRODUCED_0_'
bug.mzn:13.12-41
  in call 'forall'
  in array comprehension expression
    with i = 1
  in binary '=' operator expression
  in array access