Its a timed event, not a kill for next wave event. Sit there and they will come. Just killing the wave faster than was originally intended for the time.
It can definitely bug out, and nothing more will spawn. Might just be the instanced version. I ran into the issue when I was running some alts through it a year or so ago. Existing, although poorly described, report: https://forums.daybreakgames.com/eq...ummoner-event-agent-of-change-version.278094/
Had the same issue yesterday. After I left the AOC instance I did the event in the normal zone and completed it.
Since AoC are structured to run the raid event, not everything from the static zone goes in one. There are lots of instances where they did not copy in rare mobs/PHs. And lots of time the only zone trash are the ones that are impactful toward getting to the raid target. In this case it may be by design but the fishlord got missed in the cleanup.
The fishlord event is flagged as a raid (achievement is under "raids"), so it's counter-intuitive that the version of the zone tuned for raids wouldn't include the raid event.
That is weird, because all the gear that drops from the event is group. https://everquest.allakhazam.com/db/quest.html?quest=3919 I could make a quick guess that the person backfilling achievements saw a scripted event and figured it was a raid.
