That actually sounds like a bug to me. Checking v4's edit files, !ResistCold does have the RESIST_COLD flag, so it shouldn't be shatterable by cold attacks -- shards would be a different story, of courseu.
Well, this wasn't v4; it was Chengband, which is based on a really old version of Angband, so I'm not surprised
I'm not sure that resist cold potion should be invulnerable to cold damage. It's still just liquid in a bottle, magic happens inside the drinker after consuming it.
I think this is a case of "rule breaking" by making an exception to that rule. Question is which rule: anything giving resistance is also immune to damage caused by that element, or potions are vulnerable to cold.
[EDIT] I think temporary effects do not need to follow rule one, permanent effect items do (rings of resist lightning ignore lightning, but items making temporary effect like resistance spells in spellbook don't make book itself resistant to elements)
There's a vast difference between "this item is made of a material that intrinsically grants resistance to cold" and "this item contains knowledge that you can use to give yourself resistance to cold", much greater than the distinction between temporary and permanent effects - would you say that a book that teaches you how to build a brick wall is itself a good building material?
It's a common fantasy trope for the potions you drink to have attributes in themselves that are similar to the ones that they bestow upon you. Invisibility potions are smoky, potions dealing with heat are themselves hot to the touch, charming potions are pink and sweet, etc. In light of that I don't think it's at all out of line for a potion of resist cold to be cold to the touch and otherwise "cold-aligned", which in Angband's case means that it resists cold damage.
This make as much sense as anything I've read here, and it got me thinking about potions and their graphics.