One elemental damage replaces another. Bug?

[N5O1]

If you have additional elemental damage on your weapon and infuse gem with another one, gem effect will gem replace the first one
And I guess gems and affixes are not stacking also, even if you have same element.
Also runes and oils replacing original damage type.
it like "affix" > "gem" > "oil\rune"

And I didn't notice does "weapon oil" increases white damage when you have additional elemental damage already on your weapon?

[Tourette's Guy]

I added a fire gem to my Corpse Smeared Blade, and I think it does not deal heat damage at all. Just plague damage.