This is an old bug but every duplicate that I've seen has been resolved. From previous indications it is because the object spawns under the water and with how object-water floating is now this issue may be a lot worse as the object appears to fling itself or completely stop in its tracks. Fishing in especially deeper water with the side being a block under the surface of water appears to cause the object to stop completely ending in the same spot where the bobber was casted.