In my case, if I delete the alias and make a new one elsewhere, the alias gets the new icon, but if I put the remade alias in the same folder and I name it as the old alias, the old alias icon will be shown instead of the new one. You may not see it right away, but most probably after the finder is restarted (or reboot, or relogin).
So far, I have two simple ways to get around this bug (but still annoying). (1) If the original icon has changed and I want the alias to show the new icon, I remake the alias and rename it to a differnt name from the original alias. To simplify things, I just add a space at the end of the alias name, so the displayed name is still the same, but any other changed alias name will work. (2) If I want to change only the icon of the alias (which would then be different from the original), I paste the new image in the information window of the alias (at the top left corner), then I rename the alias. Both methods will not show the result immediately, but will eventually after some finder windows are closed and reopened later. To see the result quicker, just relaunch the Finder (in the Apple menu, choose force quit, then select Finder and click the relaunch button).