Does your pet despawn right when the gates open after the countdown, or does it happen when you accept the queue? If it is the latter, it is working as intended, and should be happening on every map. If it is the former, its a bug.
Basically. It's very annoying, it's happened quite a lot to people I play with and I don't even play with a mage all that often - if you want to make 100% sure you have a pet out just summon it after the gates have opened, even though summoning it right when you enter gives you a slight advantage far as its cooldown goes it'll always be a gamble in RoV.