I'm having the same issues. Version 1.32. Except I think I know how to even manipulate it.
First I've found I can hold infinite arrows as long as I use "give to" to shift them around between characters... but when I do something at a merchant the load comes crashing down at once and I find myself at twice my legal encumberance or more. The reason I mention that is that because when I use the give to command on arrows that are equipped and selected, it then moves the selection to the next set of arrows. When I move them back after invoking the arrow weight bug and reselect my normal arrows, I now find that they at least appear to do a different type of damage. (the type of the arrow that was in the next slot) So basicly since I found my first spiritual arrow, every normal arrow since does either spiritual or water or explosive damage.
Oh I never actually meant to exploit the arrow weight bug... I just didn't realize I was until my character suddenly became incredibly encumbered while identifying something. I had so much junk on her that it took my half an hour to figure out what it was that was so heavy.
