If you have a card that says "discard this card to ignore a bane's immunity to the Poison trait", then it's clearly not relevant to play it unless you're adding the Poison trait to a check against a Poison-Immune bane, and so is not allowed to be played in a check whilst it remains non-relevant. But the RAW state that you cannot play a card that has the Poison trait against a poison-immune bane in the first place.
I think the intention at least is for ignoring an immunity to be considered to be affecting the check. Remember that just adding a trait is considered to be affecting the check, regardless of whether that trait results in more dice immediately or even ever. It's not purely just about adding dice. You're ignoring generally immunity *for this check*, which is qualitatively different from, say, drawing or fetching a card in the hopes of doing something useful with it.
Whether that's what the rules say (yet) or not I don't know but at the very least that's how I'd play it.