This bug is easier to describe by replicating.

First create a function for a datapack:

[code]

tag @e[type=horse, limit=3,sort=nearest] add nearest
execute as @e[tag=nearest] run tellraw @a {"text":"name: ","extra":[

{"color":"blue","nbt":"CustomName", "entity":"@s"}

]}
execute as @e[tag=nearest] run tellraw @a {"text":"age: ","extra":[

{"color":"blue","nbt":"Age", "entity":"@s"}

]}
tag @e remove nearest

[/code]

note, the tags and first "execute" line were just used to make sure the target selector wasn't acting up

add the function in the rewards for an advancement with a trigger minecraft:bred_animals

when you manually run the function it properly reports the names and ages of the 3 nearest animals but when you breed 2 horses, it properly displays the names of the 3 nearest horses (the function is being called and has properly selected/tagged the three nearest horses) but the ages are all 0.