There is a slight bug with the script on the massive, bristly spider in the underground streambed.
Players trying to kill this mob are advised to initiate the fight as quickly as possible to prevent a bug that will cause the spider to disappear instead of returning to the area below its web to wait for more victims. Once the spider has been damaged once, this is no longer an issue.
Sorry for any inconvenience. Hopefully I'll find some time and motivation to fix this. Otherwise, use the mentioned workaround.
Blent