@Mush_Mush one very common form of venom is that,ofmthe enzyme... thigs other than acid can break things down, enzymes can damage tissue. Technically, it'd be digesting the tissue, but really not important. Acid/base remember is more complicated in real life than it was presented in science class, its really about molecular bonds being broken down... its really relative. My point being, something can be very resistant to acid yet not be tolerant of bases, or if the molecules are pretty big, they can be vulnerable to enzymes.
I would agree you might deserve a save bonus (ie 2 point bonus) vs toxins meant to be directly delivered to the bloodstream indtead being applied to the skin, but it shouldnt be 'ineffective'. Consider handling very hot peppers without gloves... its fine for awhile, but eventually the capsicum begins penetrating your skin, and boy does THAT burn! Capsicum is meant to be an annoyance/deterrant, and isnt very good at getting past your skin, yet given enough capsicum or some time, it still can do its job.
I think this is just one of those contradictory situations where rule-makers and game-makers don't care to know the difference about everything in play. For example, in the real world, poison is not injected (that is venom), it can either be ingested or simply touched to do it's work.
Stoneskin is described in the game as an outer skin that completely covers the caster. It is of course magical, and made for the purpose of protecting against physical attacks. It would make sense then, that something like poison, something that is not a physical attack, would still affect someone who has this spell on them.
Though, again, the people who make the rules don't pay attention to the difference between poisonous and venomous, as the OP pointed out, it was a spider did this to Neera at first. Spiders are prime examples of being venomous in the real world, not poisonous. So in that since, it should not have been able to work, as spiders need to inject, and their mandibles should not be able to pierce through stoneskin.
Sooo .... ¯\_(ツ)_/¯
I mean PnP does have different poison types but you are right in that they could have used better terminology. Type F poison for instance is used by spiders and is injected. However, Type K is used by Greater Basilisks and is considered to be a contact poison (more information on the different types of poisons can be found on table 51 of the dungeon master's guide). Though some may not know this Greater Basilisks in BG1/BGEE actually are equipped so that they could poison you if you fail a save vs death (with a +4 bonus). So technically, if only through this one example, this is a case of a poison in the game that can be considered a contact poison. You just aren't likely to encounter it (especially if you've protected yourself against their gaze) based on how the scripting works.
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I would agree you might deserve a save bonus (ie 2 point bonus) vs toxins meant to be directly delivered to the bloodstream indtead being applied to the skin, but it shouldnt be 'ineffective'. Consider handling very hot peppers without gloves... its fine for awhile, but eventually the capsicum begins penetrating your skin, and boy does THAT burn! Capsicum is meant to be an annoyance/deterrant, and isnt very good at getting past your skin, yet given enough capsicum or some time, it still can do its job.
I think a Robe gives also some Protection.
-> Poison bypass Stoneskin -> NO!