Legion has been a blast so far! However, there seems to be a lack of detailed information in certain areas. More for us to discover/learn, yes, but it can also lead to a lot of confusion and frustration.
One of these 'missing' details is the 4th Pet Battle World Quest in Suramar. I have a feeling that many may not be aware that while the 3 Pet Battle quests around the other Broken Isles zones are guaranteed, there is a 4th WQ that can sometimes appear in Suramar (it's not guaranteed to appear every day).
Not everyone will see this elusive 4th WQ, though. To access it you must complete certain Nightfallen storylines. Suramar and its content is heavily gated, and you won't get the Pet Battle WQ unless you've unlocked them first.
Unfortunately, it's not entirely clear which quest chains unlock which Pet Battle WQ. Wowhead does a great job of listing the Nightfallen quest chains and the other WQs that they unlock, but it doesn't list the Pet Battle ones.
If you have information on which quest chains grant access to which Suramar Pet Battle WQ, please let me know!
If you've completed up to and including Blood and Wine, you should be able to see Clear the Catacombs and Flummoxed, at the very least. These are the ones that I recently completed after having finished the aforementioned storyline.
Another hidden detail about these WQ is that they are on a different timer than the rest of the Pet Battle quests. I think this is due to the fact that you need to unlock them first.
So make sure you progress through the Suramar storyline, and keep checking your map to ensure you don't miss a Pet Battle WQ!