Idk which category it belong to so dumping in bug report.
Credits: Monstz
Currently this bug is giving pain to all mount players.
As you can see, the pet vanishes but you are still in mounted status and still have movespeed bonus. But you are not allowed to use mount only skills.
You were forced to relog to fix it.
So instead of relogging, you can go to rest mode (press insert) and then turn off and then on the the pet.
The pet will be back visible and you can mount again and use the skills.