Jick actually gave us the lowdown on the forum so the spading was just confirmation that he was accurate. He was.
Simply, if base damage is below 30, it is treated as if it were 30 damage for determining how much elemental resistance reduces. eg, if a player has 1 level of Hot resistance, which provides 10% resistance, and would take 5 Hot damage, it is reduced by 10% of 30, which is 3, so the player would take 2 Hot damage. Minimum damage is still 1.
Since this only affects the low end of damage, this won't affect the Basement, but it is probably good to fix anyway. At least the coding to add this minimum shouldn't be too tricky.