Many of the cosmetic mounts do not exist in the pool. It is limited (overall):
- no champ pvp bundle mount
- exclusive event cosmetic mounts
- et cetera
#Mount upgrade box glitch
radiant hamlet
outer helm
@dry plume show a pic of your mount cosmetics to explain further 👍
dry plume
Proof that I did just collect the mount box and proof I’m missing mounts
outer helm
Hard to confirm what’s missing from the list we can access (which shall not be named here), but maybe the devs can see that info.. I bet they could
radiant hamlet
@empty shale I've long since cleared the box for my Main, so I can't do any testing that way. Mayhaps the Devs 🔎 as Zombie suggested? 
outer helm
May not need a fix, but it could use a review 🤷♂️
mild saddle
The most likely reason is that you actually own all the mounts present in that box which is not all mount cosmetics in the game