Wish I could help but I have never done a small batch like this. It does seem like it comes down to the small size. I will tell you this, I have had the rubber smell in beers of mine in ehich I left the blowoff tube on my fermentor for 4 weeks.
Perhaps the small size of the batch with the same amount of surface area exposed to the rubber causes a higher risk of imparting the smell on the beer.
Makes sense in my head, but not sure I explained it well enough and again, no idea if that is right, just throwing something out there.