Nevermind, active returning tridents turn into ordinary ground items when the player is killed before retrieving the trident. Death does not cause trident disappearance, though, the time this happened in a server before might be a server problem, lag clear and such.
Update after testing: Trident also disappears while revolving due to player death. Perhaps event similar to player death is caused by server.
What version are you playing on, and if any, what mods. And what version is the server on, and if any, what mods.
How do I clear a bug report?