I salvaged a bunch of eleven woodwind long bows +1 and got no imbued elven bowstrings. Then, I salvaged a bunch of standard elven woodwind long bows (no +) and got a large amount of imbued eleven bowstrings. Some of the +1 longbows say they have strings of precision or speed (on hover over info), but they do not yield these items on salvage., Standard bows do not have these strings, yet they yield them on salvage (and at a high rate too). Edit later in the day: Just to make sure this was not a weird one time deal, i checked again with 2 smaller batches, this time i got a rare imbued string from the +1 bows, but the rate of imbued strings from the regular bow was much higher. So, I do think the "drop rate" of the imbued strings is incorrect. They should drop from the +1 bows, and either not at all or at a much lower rate for the regular bows.