The Bug

Introduced in 24w20a.

When a Carrot on a Stick breaks due to boosting a Pig, or a Warped Fungus on a Stick breaks due to boosting a Strider, it no longer returns a full-durability Fishing Rod, as it had done in 1.20.6 (24w19b) and before. Instead, the fishing rod is damaged, depending on the item it was before breakage:

*Viewing the durability of this fishing rod in an NBT editor shows the damage to be 100, so the durability of the fishing rod would be -36/64. However, it shows as 0 in Minecraft due to the fix of MC-259587.

See the videos attached.

How to Reproduce

Enable advanced tooltips by pressing F3+H to see the total durability remaining on the items.

In a 1.20.6 (or 24w19b) world, run this command for a Carrot on a Stick: 

/give @s minecraft:carrot_on_a_stick[damage=24]

Or this command for a Warped Fungus on a Stick:

/give @s minecraft:warped_fungus_on_a_stick[damage=99]

Saddle a Pig/Strider, and, in Survival mode, use the Carrot/Warped Fungus on a Stick to boost the saddled creature.

Note how the item breaks and turns into a full-durability Fishing Rod.

In a 24w21b world, run the commands above. Saddle a Pig/Strider and use the Carrot/Warped Fungus on a Stick to boost the Pig/Strider in Survival mode.

Note how the item breaks, but the Fishing Rod in its place has 25 or 100 of its durability lost, depending on if it was a Carrot on a Stick or Warped Fungus on a Stick that broke, respectively.