Fishing rod bobbers have always been an unnamed entity, and when searched for by the game, are referred to as the "unknown" entity. Using the name "unknown" in the type= field of a target selector allowed players to target fishing bobbers anyways.

However, in the 1.11 snapshots, although still referenced as an "unknown" entity, target selectors will no longer accept "unknown" as a valid entity ID. This is likely due to the renaming of Entity ID's back in 16w32a. Fixing this bug would involve simply giving the fishing rod bobber a proper ID.