nafc doesn't work with more complex goals

The negation as failure constraint nafc is supposed to succeed if and only if the goal provided by a relation and its args fails. The test cases just cover very simple cases like (nafc == q 'b) which is essentially equivalent to (!= q 'b) (at least to my understanding). But with a slightly more complex case, it doesn't seem to work anymore.

Example:

The constraint specifies that the number 2 must NOT be contained in the list q but still it is. I expected to get the single answer (1 3) here.