OK, to test it out I made another Bag of Holding and true enough, they are NOT cumulative. The second bag shows exactly the same stuff what's in the "first" bag.
However, scroll cases are different, they are cumulative, as well as gem bags and such. And also true, every item is counted as separate, so 5 scrolls separately takes just as much space as 5 scrolls stacked. Which in a way is corny, at least regarding the stacked items.
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