The recent patch appears to have introduced a new pet summon bug. It will cause a pet to sometimes appear after a summon with no pet action bar and be unresponsive to commands.
The exact nature of the summon bug has yet to be determined to my knowledge. I am able to replicate it consistently by flying into an instance (I used The Stonecore). Simple Call Pets, dismounts, etc., do not reliably trigger the bug if at all. In other words, the bug is not a consistent problem (and possibly not a problem at all) for most kinds of pet summons. It is, however, an issue for people queueing into dungeons and BGs, flying into raid instances, and other situations where the player is on a flying mount during instance zone-ins.
While the pet is bugged it will be unresponsive to commands like /petattack in addition to missing its action bar. The pet’s tab in the Spellbook (where all the pet abilities are) will also be missing. The condition very much resembles the state that pets are in for new hunters, where the pet just ignores anything the hunter tells it to do and stays on Assist the whole time. Bugged pets appear to be much the same, retaining whatever stance they were in before they got bugged and ignoring instructions.
To fix the bug, simply resummon your pet in some way. Zoning out, Dismiss Pet plus Call Pet, etc. appear to work. Reloading the UI also works, suggesting that this is a UI issue rather than some issue with pet mechanics. It also adds further weight to the possibility that this is an improper activation of the pet control restrictions placed on young hunters. Note that while bugged a pet will respond to the /petattack command built into Kill Command; so if you zone into combat you can at least get your pet to attack things that way if it’s stuck on Passive.
To avoid the bug, try to not be on a flying mount when accepting queues, summons or otherwise moving in or out of an instance.