First time this happened to me, there was no one else around. Waited for others to show-up to see if it was just me or not (wasn't just me).
After going away and doing other quests, I came back and visited the pool. No one else around, but I was able to interact with the pool and finish that quest.
Basically, that makes me think the bug isn't a re-spawn issue. However, people standing around might prevent it from working when revisiting later.