Horse armor with a stack size greater than 1 (possible through the max_stack_size item component) cannot be placed in full onto a horse using the horse's inventory screen; only a single item in the stack can be placed. However, with a similar saddle with a stack size greater than 1, the whole stack will be placed. This is an inconsistency and does not seem like correct behavior.

(The saddle slot does not accept oversized stacks using shift-click, which furthers the impression that this is a bug.) This seems to be no longer true since at least Minecraft snapshot 24w34a.

The following commands can be used to obtain items to test the bug:

• /give @s minecraft:saddle[minecraft:max_stack_size=99] 99
• /give @s minecraft:diamond_horse_armor[minecraft:max_stack_size=99] 99

The following command can summon a tamed horse to test the bug:

/summon minecraft:horse ~ ~ ~ {Tame:1b}

Most related entities (donkeys, mules) are affected. Llamas are not affected.