So, I got it to pop. Someone earlier recommended uninstalling the game, going offline, reinstalling from disc while staying offline, and see if that worked. I went to the save right next to the albino spider, saved, then quit and did the whole delete, offline, reinstall thing. Like someone earlier, I was pretty certain that I saw that thing drop the first time and ran from it, but it was never there when I came back. I think I may have gotten the scan from it in the bounty hunters arena, since I seem to recall fighting one there, too. Anyway, I jumped into that spot when the game installed offline, and sure enough, the spider was there. I killed it, saved at that same mediation spot, quit the game, and went back online. I installed the update, loaded back in, and the albino spider was gone. Curiously, my save game slot dropped from 100% to 97% in this process, yet everything is complete. Anyway, the achievement didn't pop right away; I returned to the Mantis and traveled to another planet, and then meditated on the ship, and that's when it popped.
Regardless, it's an obvious bug (er... spider?). An undefeated beast shouldn't just disappear and not give you the chance to beat it without resorting to such lengthy measures. Would really suck if I didn't have the disc to install from.