Many (though not all) mounts from the opposing faction are no longer being returned by the character collection API. With the API rework it was intended for the entire account’s collection to be returned for any player, but this is no longer the case.
Looking at the list of mounts that I own that aren’t appearing, I can’t see a defining factor. Since there are so many missing it’s easier to define which do still appear:
- racial mounts still appear
- the original “Black War” pvp mounts still appear (the later Black War Mammoths, Black War Bears, etc. do not)
- the pair of mounts from the “Two Sides to Every Tale” achievement still appear
But not much else…