From the patch notes:
Arena Weapons- Caustic Arrow: Fixed an issue where this set was not retaining its effects after bar swapping. As a result, this item set is no longer able to be handled by the server entirely, and as such will no longer work in no-proc environments.
So ZOS, you are telling us that your servers suffer from short-term amnesia and therefore you disable the weapon altogether? Seriously, how low can you go with your server performance?
That for sure is a lame change if I ever saw one.