I’m not familiar with the finite domain work.

But if you change the fd/in to

(l/membero fx [0 1 2 3 4 5]) (l/membero fy [0 1 2 3 4 5]) (l/membero fz [0 1 2 3 4 5])

it works

So I suppose either there is a bug in fd/in or you are not using correctly here.