The bug

Unlike the majority of other entities, spawners set to spawn fishing bobbers do not render the fishing bobber inside of themselves.

How to reproduce

1. Run the following command:

   /setblock ~ ~ ~ minecraft:spawner{SpawnData:{entity:{id:"minecraft:fishing_bobber"}}}

2. Look into the spawner
3. Try and find a small, rotating (if not in Spectator) entity model

Expected results

A small fishing bobber would be visible inside of the spawner.

Actual results

There is no such model.

Further notes

As fishing bobbers are an atypical entity (they require an associated player), and they produce errors in the game log due to not having any such associated player as well as ultimately not really spawning anything, fishing bobber spawners probably aren't something that make sense to be complaining about, so this could very well be considered Invalid unless there's something else that can be done with NBT I haven't accounted for (such as having the fishing bobbers created by the spawner specifically assigned to a given player so they don't just immediately stop existing). If anyone knows of any ways to do this, please comment.