This problem was introduced in 24w05a due to the change that horse armor is stored in body_armor_item instead of ArmorItems[2].

The fire protection enchantment on horse armor does not shorten the duration of the horse's burning status. Nonetheless, it does mitigate the damage caused by fire. This mirrors the issue described in MC-269780, which remained unreopened even after more than a week.

Note: Entities exposed to lava catch fire for 300 ticks (equivalent to 15 seconds), irrespective of their duration of immersion in the lava.

Steps to reproduce

1. Obtain a tamed horse
2. Ride the horse
3. Outfit it with horse armor enchanted with Fire Protection IV
4. Swiftly deploy lava to ignite the horse, then promptly remove it

5. /execute on vehicle run data get entity @s Fire

Observed: The command returns nearly 300 ticks, suggesting that the duration was not diminished.
Expected: The remaining duration of the horse being on fire should be approximately 120 ticks, as Fire Protection IV reduces the duration by 60%. Thus, it should be aflame for 120 ticks instead of 300.