In a Joule expansion, which is an irreversible adiabatic expansion against a vacuum, $q = 0 \text{ and } p_{ext} = 0$. Thus, since the only type of work in a Joule expansion is $pV$-work:
$$\Delta \text{U} = q + w = w = -p_{ext} \Delta V = 0 \text{, always.}$$
And since $\Delta \text{H} = \Delta \text{U}+ \Delta pV$:
$$\Delta \text{H} = \Delta pV \text {, always.}$$
For ideal gases:
$pV$ = constant (at a given $T$), so $\Delta \text{H} = \Delta pV = 0.$
For real gases, we have two cases:
I. At the inversion temperature:
$pV = \text{constant (at a given } T \text{), so } \Delta \text{H} = \Delta pV = 0$.
II. Not at the inversion temperature:
$pV \ne \text{constant, so } \Delta \text{H} = \Delta pV \ne 0$.