Most items that you are not allowed to use in the offhand simply don't let you use them, but trying to use the fishing pole plays the sound and you see it cast for a split second before it disappears. The expected behavior is that when you right click nothing happens.

Steps to reproduce: press 'f' while holding fishing pole to put it in your other hand, then right click to use it.

This is similar to MC-84536, but that issue was marked as resolved.