On a completely vanilla server with two clients (no client-side mods), the fishing rod's bobber and line are not visible. The fishing rod is still able to pull entities and makes the two distinct sounds for casting and reeling as expected, but is otherwise invisible. Both the host (connecting via localhost) and the other client (connecting over internet IP) are unable to see the line/bobber, regardless of who cast the line and which fishing rod is used (enchanted rods are also invisible).

No other entities or effects are noted to be invisible on either of our clients, and this bug was not present in our server and clients of previous Minecraft versions.