Hey all,
I can confirm that our developers are currently investigating this issue, which appears to affect the display of faction-locked mounts (these will not appear in your Collection, instead of appearing as unusable/locked).
You can also find more information about known issues and their resolution on our Blizzard Support Discord server.
Bug fixes are often posted in our Hotfixes and Updates blogs, in the News section of the site. If you encounter another bug in-game, please report it to them, and they will be able to review it for you.
Sorry for the inconvenience, and thank you for your patience!