Poisoned weapons/missiles seem to have an amount of poison, which wears off hit by hit, until they are no longer poisoned. (See also: http://www.adom.de/forums/project.php?issueid=3403)
So, despite the appearance in inventory, these aren't all the same item: they're arrows with (say) 3, 2, and 1 hits of poison respectively.

Which is fine in theory, but horrible in practice, because missile stacks breaking down into heaps of 1-2 make you waste a turn reloading every shot or so. I wonder if we could have a fix that would make them appear in the inventory as a single stack but still keep different amounts of poison on them (and give an appropriate message when a missile runs out of poison and turns normal again, so you don't think it disappeared). It probably requires either a lot of work or a couple of nasty fixes in the code though.

Wouldn't it be better to just make arrows poison their target once, after which the poison wears off?
That way you don't have to introduce some new complex mechanism of action to deal with just a bunch of poisoned arrows but instead you have two stacks - still poisoned and not poisoned.
For a sword or a spear, I understand the concept of poison acting more than once - it's a big piece of metal and you can wound and poison multiple enemies with it.
However, for an arrow/quarrel, it's a small amount, probably just enough for a single shot.

I'm missing the quiver feature again...

Not just you.

Not just you and you.

Really. One (athough not very simple) feature to fix all the issues with stacking ammo once and forever.

TB, The Creator, please hear us, consider quivers for implementation once again. This will make archery in ADOM awesome!

From RFE - Simplify stacking of poisoned ammo:
Problem: When you dip a stack of ammunition into poison then fire it and pick it up again, the ammunition no longer stacks.
Desired solution: Poisoned ammo should continue to stack for as long as it is poisoned.

And getting into theorycrafting a bit:
Hypothesis: Items currently track how much poison is left on them, or something similar.
Proposed solution: Make items be 'poisoned' or 'not poisoned', and assign a % chance for poisoning to go away completely.