The Vashj’ir Seahorse (mount ID 373) is no longer appearing in mount collections in /profile/user/wow/collections/mounts (but confirmed it’s showing as collected in the in-game journal).
I’m also not sure when this started. The first report I received was a few days ago.
It is appearing in character collections on the official site (for example, Wain - Mounts - Collections ) so I’m not sure what the difference is (in the past, player collections on the official site seemed to use the same data and show the same bugs as the API).