The fishing rod seems to have visually lost its hook slot whilst in hands.
The slot is visible if the rod is in the vacinity when you open your inventory.

The 1.14 fishing rod:

1.15 expie fishing rod:

And 1.15 on the floor:

The combine option is still available in 1.15, but the hook doesn't show on the rod whilst the rod is in hands, and you open your inventory.

We've also noticed some issues whilst actually fishing too, we'll append that to this ticket as we find out more.

So far:

• Normal rod does not extend whilst fishing
• The chance of catching nothing has been noticed by some players, one catching nothing 3 times in a row despite a pristine rod and baited hook. (Perhaps a Sea loot item is bugged and landing under the map? or not spawning in at all?)