This happens if you favorite the Chittering Animite mount and use the Summon Random Favorite Mount button.
If the Chittering Animite is the ONLY mount, the Summon Random Favorite Mount button will summon that mount.
However, if you pick ANY additional mount to add to the list of Favorited mounts, it will never select the Chittering Animite.
Did a bit more testing. It will appear normally if the Random Mount button is used in any other expansion.
My theory has changed, the bug may be connected to how it’s considered a “flying” mount and since in the Shadowlands, you cannot fly, that MOUNT then isn’t an option. Similar to how Underwater mounts don’t pop up on dry land.
However, if you click on that mount specifically, it will be usable anywhere in Shadowlands, just not in the Random Mount button.
This makes sense. The “random” button is disqualifying flying mounts in ground-mount-only zones, although you can still use flying mounts as ground mounts. The logic behind the button is needlessly selective.
If Blizzard doesn’t fix this (either because it can’t get prioritized enough or because they actually prefer it to work this way), this would be easy enough to do with an addon instead of Blizzard’s built-in capability. The “random mount” button was originally an addon anyway; as usual, Blizzard co-opted the capability into a built-in but did an inferior job of it.