Molten Shell explodes while Immortal Call is active

This is correct and intended behaviour. Molten Shell technically counts any physical damage that's mitigated by anything, including immortal call - which mitigates all physical damage. Immortal Call is in fact sometimes used to make Molten Shell go off faster, since more damage is mitigated per hit than just armour.

The current description on Molten Shell lists the most common forms of physical damage mitigation, because talking about "mitigation" without specifying what type confuses players as to what it's supposed to do, whereas they understand what they're armour and block chance are, and that those operate separately from Molten Shell, and can thus draw the (mostly) correct conclusion about how it works.