Spawner's entity display defaults the values of NBT data to basic value, rather than mobs' default.
Here are some examples:
Goat in the monster spawner has no horns.
Skeletons, strays and wither skeletons in the monster spawner has no weapon.
This is basically the same as spawn mob using "/summon <mob>", and the spawner display using "/summon <mob> <x> <y> <z> {}"; this is probably because the spawn data is required to have the id set.
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Yea, I know; that's what confuses me. The same concept can be seen with a skeleton spawner. The skeleton doesn't appear to be holding a bow in the spawner and yet spawns with one upon being summoned.
My guess is that the entity's model within the spawner uses the said entity's default NBT data if no NBT data of the entity has been specified.
I think this report should be redefined as "spawner's entity display defaults the values of NBT data to basic value, rather than mobs' default", and clarify in the description that skeletons', strays', and wither skeletons' weapon and goat horns are examples of this occuring.
This is basically the same as spawn mob using "/summon <mob>", and the spawner display using "/summon <mob> <x> <y> <z> {}"; this is probably because the spawn data is required to have the id set.
I'm pretty confident in saying that this behavior is intentional, though that isn't my call to make. Regardless, I can confirm this behavior.
Basically, whether a goat has horns or not is controlled by the "HasLeftHorn" and "HasRightHorn" NBT tags, and by default, these tags are both set to "0b". If you want goats to have horns inside of spawners, you'll need to explicitly specify this. For example, the below command will place a monster spawner at your position that will only summon goats with their left horns, and as a result of this, the model of the goat within the spawner will only render its left horn.