The bug
When replacing a cast fishing rod in your inventory with another fishing rod (with the same NBT data), then the fishing rod shown in your hand in first person might be displayed as if it is not cast.
Ways to reproduce
Selecting other one
Place two fishing rods next to each other in your hotbar
Cast one
Scroll to the other
Replacing using hotbar keys
Place one fishing rod in your hotbar and another one anywhere else
Cast the fishing rod in your hotbar
Open your inventory
Hover over the other fishing rod and press the hotbar key of the first one (e.g. if it is in slot 1, press
1
)
Might be fixed in the next snapshot, see MC-171484