I was unable to find an active bug report after searching for this issue, even though multiple people have mentioned it to me as of lately. This is something myself, along with other players are experiencing. My experiences are on a BDS multiplayer server. I have not tested if this takes place in a single player world.

This is what is happening.:

The Tridents in all trident killers are falling through the blocks that they sit on.
The Tridents will fall down and destroy the hopper minecart used for the collection system directly under the Trident killer. Sometimes the Tridents will even fall through the hoppers, making 2+ blocks that they fall through. This makes any farm that uses a Trident killer un-usable until the tridents are reloaded into the Trident killer. This seems to only occur when the chunks are reloaded. Either by leaving and returning to the game, or by leaving the area. 
Note that the trident killer does not have to be on with the trident moving for this to occur.

expected behavior:

The trident should be exactly where it was left when the game was unloaded.