I'm occasionally running into bundles of arrows that have 34 arrows in them. I thought BG1 only allowed 20 arrows. Did the expansion add these in, in preparation for BG2? Or is it some kind of bug?

It is a minor bug. If you ever unstack them they will never revert to a stack larger than 20.

But - if you put two stacks of more than 20 arrows together you'll get one with 20 and one with the rest (I've gotten around 80 +1 arrows in a single stack - a nice little reserve)