All sling/bullets, and darts get Str bonus, Dex bonus too. They always have I'm pretty sure.
They did, but it was patched out recently I believe. Either way there's an inconsistency in the behaviours with other bullets. I'd personally give them all the strength bonus per AD&D rules, since these are hurled, but the BGEE devs disagree, and since they make the game and I don't, then it means the bag of plenty +1 needs to lose the strength bonus.
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I had a sorcerer with 16 str that was getting constant 1-3 damages on darts.